Montreal authorities expelled 40 people and detained 34 more from the site of the Grand Prix on Sunday, calling the move a "preventative measure."

Police said those who were detained or expelled from Ste-Hélène Island were carrying objects including bricks, rocks or ski masks.

"Some of those arrested were people police recognized from earlier student demonstrations that had been deemed illegal," district commander Alain Simoneau told reporters. "In the interest of public safety, we decided to detain these people."

Some people arrested in neighbouring suburbs could face charges, Simoneau said, but most arrested near the event will not be fined or charged. Those who were detained were taken back to Montreal, while the others were made to turn around and reboard the metro.

The search and detention operation at the Grand Prix race site began just after noon, near the exit of the Jean-Drapeau metro station, as an estimated 100,000 people — not including Premier Jean Charest, who steered clear of the event — made their way to the island for the big race.

Police searched scores of people on their way to the event, citing security concerns.

The race got underway at 2 p.m. ET at the Gilles-Villeneuve track.

Tight security for race

There was heavy police presence at key public transit points and Grand Prix events across Montreal, including the main race site, after an anti-capitalist group called for disruptions. Hundreds of police officers and sniffer dogs swarmed the underground train line that services the track site on Ste-Hélène Island.

The Canada Grand Prix weekend event was marked by tense standoffs between protesters and police. (Canadian Press)

Police conducted random searches of metro riders, with some people being turned away from the train platform. Those who didn't have a ticket for the race, including some reporters and some planning to purchase a ticket at the gates, were not allowed onto the island and were ordered back on to the subway.

Many of the people turned away were wearing red squares, the emblem of the student movement.

The added security comes less than 12 hours after a chaotic downtown protest that resulted in dozens of arrests.

On Saturday night, 28 people were arrested as protesters tried to push into the Crescent Street bar strip, where several Grand Prix parties were being held. About 16 of those arrested Saturday will face charges, according to police.

At least four police vehicles were vandalized — including one that was completely flipped over — and at least one business had its windows broken.

The Grand Prix has been targeted by demonstrators protesting student tuition-fee increases and capitalism in general.

Bomb threat shuts metro

On Sunday morning, police arrested a man after a bomb threat along the yellow subway line that services the Grand Prix race site.

Police received a call at 7:50 a.m. about a possible bomb at the Longueuil/Université-de-Sherbrooke station.

The bomb threat turned out to be false, but it still provoked a three-minute service disruption on the metro line.

Police tracked the suspect, a 40-year-old man, to his home where he was taken into custody.

In another plan that targeted the transit system, an anti-capitalist group organizing Sunday's protest suggested cramming the subways to slow down fans heading to the event.

The signal to board appears to have been a fire alarm, which was set off on the subway line Sunday morning. Several people were removed from the metro following that incident.

With files from the Canadian Press