Syrian security forces fired tear gas and live ammunition to disperse thousands rallying Friday in Aleppo in what activists said was the largest protest yet in a city that has largely remained loyal to President Bashar Assad during the country's 15-month uprising.

Thousands of people across the country also staged anti-government rallies in solidarity with Aleppo, where anti-regime sentiment has been on the rise particularly after a raid on dormitories at the city's main university killed four students and forced the temporary closure of the state-run school earlier this month.

The May 3 raid at Aleppo University was an unusually violent incident for the city, a major economic hub that has remained largely loyal to Assad over the course of the country's 15-month uprising.

On Thursday, some 15,000 students demonstrated outside the gates of Aleppo University in the presence of UN observers, before security forces broke up the protest.

Even bigger numbers took to the streets Friday. Aleppo-based activist Mohammad Saeed said it was the largest demonstration there since the start of the uprising. He said more than 10,000 people protested in the Salaheddine and al-Shaar districts alone and thousands protested in other areas of the city.

"The number of protesters is increasing every day and today saw the biggest protests," said Saeed, adding that several people were wounded when government forces fired tear gas and live ammunition to try and disperse the rally.

The head of the British-based Syrian Observatory for Human Rights, Rami Abdul-Rahman, said the protest showed "it's a real uprising happening in Aleppo these days."

Friday is the main day of protests across Syria and this week's demonstrations were dubbed "The Heroes of Aleppo University" in solidarity with the students.

Only 'genuine' talks can end violence

Opposition activists said security forces opened fire in several other locations including the Damascus suburbs and the central city of Hama to disperse protesters and that the regime shelled the town of Rastan, which has been under the control of rebels since January.

The violence comes as the head of UN observer team in Syria cautioned that unarmed force alone cannot stop the bloodshed without genuine talks between the two sides that have been locked in a violent conflict for more than a year.

Maj.-Gen. Robert Mood, the Norwegian head of the 200-strong observer team, warned Friday that no number of observers can achieve "a permanent end to the violence if the commitment to give dialogue a chance is not genuine from all internal and external actors." He spoke at a news conference in the Syrian capital, Damascus.

International powers have pinned their hopes on a peace plan for Syria that special envoy Kofi Annan brokered in April. The plan paved the way for the UN observers, and it calls for a ceasefire and dialogue to stop 15 months of bloodshed.

The UN estimated in March that the violence in Syria has killed more than 9,000 people. Hundreds more have been killed since then as a revolt that began with mostly peaceful calls for reform has transformed into an armed insurgency.

Both sides have flouted the ceasefire, raising concerns that the peace plan is ineffective in a conflict where the violence is spinning out of control.

Ahmad Fawzi, spokesman for Annan, said in Geneva that the envoy would be visiting Syria soon, but did not give a date.

"We condemn in the strongest terms violence in all its forms by all parties," Fawzi said. "This must come to an end for any political process to be launched and to have a glimmer of success."

A high-ranking military adviser to UN Secretary Ban Ki-moon, Babacar Gaye, arrived in Damascus Friday.

But dialogue seems a distant hope. The opposition refuses to engage with the regime while the killings continue, and the government brands its opponents as terrorists.

On Friday, Foreign Affairs Minister John Baird announced further sanctions against Syria, banning the export, sale or shipment of luxury goods to the country. "The ongoing, appalling violence perpetrated by the Assad regime against the people of Syria compels Canada to further isolate the regime and its backers," he said in a release.

Canada also added six more names to a list of people and organizations subject to an asset freeze and prohibition on economic dealings.

UN chief believes al-Qaeda is in Syria

Assad denies that there is a popular will behind the country's uprising, saying foreign extremists are driving the unrest to destroy the country.

Two main activist groups, the Local Coordination Committees and the Observatory, said Rastan again came under intense shelling as of Friday morning. Videos posted online showed thick smoke and shells slamming into districts in Rastan.

"I am more convinced than ever that no amount of violence can resolve this crisis," Mood said in Damascus. He also said recent suicide bombs and roadside blasts were alarming. "I am concerned about the incidents where explosives, improvised devices are targeting innocent civilians, innocent people because it is not going to help the situation."

On Thursday, UN Secretary-General Ban said he believes that "alarmingly and surprisingly," al-Qaeda must have been behind the massive attack in the Syrian capital last week.

The twin suicide car bombings outside a military intelligence building on May 10 bore the al-Qaeda-style tactics seen in neighbouring Iraq. Some 55 people died and dozens were injured in the Damascus blasts.

Ban said that al-Qaeda's involvement in the region "has created very serious problems."

He also noted that there have also been two attacks against unarmed UN monitors in Syria. His comments were made to students attending the annual Model UN Conference in New York.

With files from the CBC