(CNN) -- Tamil demonstrators protesting the ongoing civil war in Sri Lanka flooded onto a major highway in Toronto, Canada, shutting it down for several hours before dispersing early Monday, police said.
There were no arrests, said Sgt. Chuck Konkel of the Toronto Police Department, who estimated the crowd's size at 500 to 1,000 people, although some put the number at several thousand.
It was not clear what the protesters intended by blocking the Gardiner Expressway -- "a very significant arterial road for the city of Toronto," Toronto Police Chief William Blair said. Members of Toronto's Tamil community were taking part in a peaceful demonstration in a nearby park when they moved to block traffic.
"They gave us no indication they were going to do what they [did]," Blair said.
Many of the protesters were on an overpass, and he said many children were in the crowd. Blair called it "an unsafe situation" before the demonstration broke up shortly after midnight.
In Sri Lanka, the military has been on an offensive to try to finish off the ethnic Tamil insurgency that has been fighting for an independent state in the northeast of the country since 1983.
As many as 70,000 people have been killed since the civil war began, and the Tamil Tigers rebel group has been declared a terrorist organization by the European Union and more than 30 countries, including the United States.
On Sunday, a humanitarian worker in Sri Lanka told CNN that nearly 400 people were killed overnight Saturday by government shelling in what is supposed to be a "no-fire" zone. Most of the roughly 378 dead were civilians, and an additional 1,200 were wounded, the aid worker said.
The government denies the claim, saying it is the Tamil Tiger rebels who have been killing civilians.