Tamil demonstrators lock hands under the eye of police Friday as they block the entrance to Toronto's Union Station. (Robin Rowland/CBC)

Thousands of protesters flooded onto one of Toronto's busiest downtown streets at rush hour on Friday in opposition to a Sri Lankan government offensive aimed at crushing the separatist Tamil Tigers.

The protesters formed a human chain outside Union Station, a main transit point for commuters catching trains, subways and buses out of the downtown core.

The flood of protesters and commuter traffic clogged the area as the rally began to disperse at 6 p.m.

The protesters gathered along several kilometres of slushy sidewalks in the downtown core beginning around noon, chanting slogans such as "we want peace" and "help us, Canada". They remained in the area as commuters tried to reach Union Station at the end of the workday and police directed pedestrians and traffic through the area.

Protesters say that in recent days, more than 800 Sri Lankan civilians have been killed in the offensive while government officials say the number is closer to 300.

Scores of young people, off because of the province's high school exam period, participated in Friday's protest, carrying signs and chanting for an end to the violence.

"This is a massive protest, because we're expecting the whole Tamil community to come out and support this," said 17-year-old Phavalan Rahendram.

"It's not only Tamils that should care about this," Rahendram said. "This is the killing of human beings. This is a genocide."

The protesters say their loved ones are caught in the middle of a deadly battle between the Sri Lankan government and Tamil rebels.

Ragavan Baladurai said he has just learned that his aunt has been killed in the conflict.

"She was the one raising me, so that's one of the reasons I'm here," he said while attending the rally.

Shya Thava said her family has been waiting to hear from relatives and friends caught in the crossfire.

"Every time we get a long-distance ring tone, we think it's somebody from back home at least calling us to let us know they're OK, but that call never comes," Thava said.

Thava said she hasn't heard from any family members for more than a month.

"We're out here to tell Canada to take a stance with us," she said.

About 200,000 Sri Lankan Canadians live in the Toronto area, and the city is home to one of the largest Tamil populations outside Sri Lanka.

A number of protests have been staged in recent days in the city, including rallies outside the Sri Lankan and U.S. consulate on Thursday.

Rebels have been fighting for a separate homeland for ethnic Tamils in northern Sri Lanka since 1983. More than 70,000 people have been killed in the civil war.

The rebels were ousted from all major towns after heavy battles in recent months and are now cornered in a 300-square-kilometre area of jungle and villages in the northeast.

The 25-year war of separation between Sri Lanka's Singhalese Buddhist-dominated government and the Hindu Tamil Tigers is believed to be close to an end.

Sri Lanka, however, ruled out a ceasefire with the Tamil Tigers on Friday, despite growing reports of casualties among civilians trapped in the war zone.

A humanitarian crisis is unfolding, and aid agencies are struggling to get into the area.

The Red Cross estimates 250,000 civilians have fled to the jungle where fighting is taking place.

The Tamil Tigers were declared a terrorist organization by the U.S. in 1997 and by Canada in 2006.

With files from the Canadian Press