About 150 anti-Trump protesters marched along the streets in an unscheduled demonstration Sunday in downtown Cleveland, a peaceful event that law enforcement officials likely hope will be the standard over the course of the next four days.

Slowly followed by dozens of officers on bicycles, the demonstrators, many who supported the Black Lives Matter movement, held up large banners that read "Stop Trump and the RNC" and "Stop Trump and stand against racism."  And when it was over, the crowd quietly dispersed.

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Police on bicycles wearing protective garb stand along the sidewalk during an anti-Trump protest in Cleveland. (Mark Gollom/CBC)

Police Cmdr. Keith Sulzer said the protesters did not have one of the official protest permits that have been handed out to dozens of groups who have come to the city, the site of the Republican National Convention.

Still, police allowed them to march. "We're letting them voice their concerns and that's completely fine," Sulzer said.

There were other peaceful rallies on Sunday. Circle the City With Love gathered hundreds at the Hope Memorial Bridge to spread a message of unity and anti-violence. And dozens of women posed nude near the arena as part of an art project.

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Women pose nude for photographer Spencer Tunick's art installation Everything She Says Means Everything near the location of the Republican National Convention in Cleveland on July 17, 2016. (Lucas Jackson/Reuters)

It remains to be seen whether police will be as permissive about such protests as the Sunday march when the convention officially kicks off on Monday, when concerns of violence will be heightened. Thousands have poured into the city for the convention, and thousands more, representing various causes, are preparing to march and demonstrate.

"Let's put it this way: our son is volunteering and I'm comfortable enough with what I've seen to have him down here," said Cleveland resident Rich Dugan, who, with his wife, was among the scores of delegates, volunteers, media and police milling around the popular downtown East 4th Street district. "Plan for the worst and hope for the best. It's not really kicked into full gear yet, so we'll take one day at a time."

Cleveland Police Chief Calvin Williams has said law enforcement officials tasked with overseeing the security of the city during the four-day Republican National Convention are prepared for "anything and everything."

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An impromptu Black Lives Matter protest took place in downtown Cleveland late Sunday afternoon, a day before the Republican National Convention officially kicks off. (Mark Gollom/CBC)

Williams said the force has gone through the "what if" scenarios, the "table top" exercises — security sessions discussing the roles, responsibilities and responses to any given situation — and ultimately "have planned for everything that can and will happen."

The RNC, at Cleveland`s Quicken Loans Arena, brings together delegates from across the country to vote on the party platform, hammer out party rules and officially choose the party's presidential and vice-presidential candidate.

Conventions have often been the scene of violent protest, and that concern has been amplified by the tension across the U.S. in the wake of high-profile police shootings of black men, which have led to demonstrations and clashes with police.

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A security official on horseback rides through downtown Cleveland. Around 3,000 federal officers will provide support for the convention, in addition to police from the local and state levels. (Spencer Platt/Getty Images)

Security issues have also been heightened following the shooting of five police officers in Dallas by a sniper and the recent attack in Nice, where more than 80 people were killed when a man driving a truck barrelled down a crowded street. 

'Concerned about the possibility of violence'

Then on Sunday, three Baton Rouge police officers were shot to death and three others wounded.

"It always heightens your senses, and we always know it's something that we can face," said Sgt.Scott Louive of the Ohio State Highway Patrol. "Our goal is to just make sure that everybody is safe." 

There is also worry that Donald Trump supporters and protesters, who have clashed at other rallies, could precipitate more violence.

"I am concerned about the prospect of demonstrations getting out of hand," Homeland Security Secretary Jeh Johnson told the House homeland security committee this week. "I am concerned about the possibility of violence."

FBI Director James Comey, speaking at that same hearing, added that there is always concern that an event like this will attract people from across "a spectrum of radical groups."

Along with local officials, dozens of federal agencies — including the U.S. Secret Service, the Department of Homeland Security and the FBI — have been and continue to be involved in security detail.

The city was given a $50-million grant to boost security specifically for the Republican convention. Officials have been wary to provide information on security, but anywhere between 4,000 to 5,000 law enforcement officers including state, local and those from other jurisdictions will be on hand for the event. Officers have undergone hours of comprehensive training, Williams said.

Extra equipment includes 2,000 sets of riot gear with body armour and batons, 2,500 steel barriers and 16 police motorcycles, CNN reported. Meanwhile, security cameras have been placed throughout the city.

"Trust me. There will be enough video coverage both on the ground and in the sky," Williams told reporters.

Around 3,000 personnel, including members from the U.S. Secret Service, Homeland Security and the Coast Guard, are dedicated to the security of the convention itself, Johnson said. 

On Friday, officials set up concrete traffic dividers and tall metal fences around the convention site, propelled by fresh urgency in the wake of the Nice attack.

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People attend a protest in downtown Cleveland on July 16, ahead of an estimated 50,000 people arriving for the RNC. (Spencer Platt/Getty Images)

Security experts said that the U.S. Secret Service and other law enforcement agencies have viewed vehicles as a potential threat since early in their 18 months of planning for the convention.

A 4.4-square-kilometre event zone has been established in downtown Cleveland, with protest groups being assigned permits and being limited to certain areas. Some who never sought a permit, however, have suggested they may demonstrate in other areas.

No tennis balls, but guns OK

Security officials drew up a list of prohibited items within the event zone that include items as diverse as water and pellet guns, knives, sledgehammers, ladders, coolers and ice chests, canned goods, umbrellas with metal tips and tennis balls.

But Ohio is an "open carry" state, meaning gun-owners will be able to carry firearms, except within the security perimeter that surrounds the convention venue, dubbed "The Q."

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A U.S. Secret Service agent closes a security fence amid preparations for the four-day Republican National Convention in Cleveland, which kicks off on Monday. (Associated Press)

This has raised some concerns, as there were reports that certain groups, like the Oath Keepers,  may come armed. But the national president of the organization denied this.

But not all are satisfied with the security detail. Steve Loomis, president of Cleveland's police union, has urged Ohio Gov. John Kasich to suspend open carry laws for the entire state during the convention and to declare a state of emergency.

Call for suspension of open-carry laws

"I don't care what the legal precedent is, I feel strongly that leadership needs to stand up and defend these police officers," he said in an interview with Reuters on Sunday, following the shootings in Baton Rouge.

Kasich's office responded to the call, saying he does not have the power to suspend the laws.

Loomis has argued that his members are not prepared for the convention and that officers were not getting equipment on time — or receiving the proper training for it.

​Cleveland's police chief dismissed Loomis' concerns. 

"I don't think rank-and-file officers share the union president's sentiment," he said.

Securing the RNC - By the numbers

With files from The Associated Press, Reuters