The Canadian Grand Prix started under heightened security on Sunday after student protesters had threatened to disrupt the Formula One race.

Race fans arriving at the Circuit Gilles Villeneuve under brilliant sunny skies were met by a heavy and vigorous police presence at all entry points to the island track which hosts the seventh round of the season.

Security efforts were primarily focused on Montreal's subway system, which provides the main link to the island, that protesters had threatened to overload.

A bomb threat briefly shut down the system on Sunday but caused no major delays as close to 100,000 Formula One fans made their way to the race track.

Closer to the 2pm (1600 GMT) start, however, CBC radio reported that tensions were running high on the metro.

The organizers' biggest worry is that protesters might follow through on threats to invade the track, prompting beefed up security around the entire 4.361 km layout.

The sometimes violent student strike, which began in mid-February after the Quebec provincial government announced plans to raise tuition fees, has taken on a wider scope with groups opposed to global capitalism, anarchists and labor unions joining in.

Protesters have made their presence felt during the four-day festival, which is among Canada's biggest tourist events, with noisy nightly run-ins with Montreal riot police that resulted in several arrests but no major incidents.

Before Sunday, protesters had targeted the many events connected to the race and particularly the area around the city's Crescent Street, which served as party central for the Formula One weekend.

They taunted well-heeled revelers and pushed their way into the avenue's posh bars and restaurants. The mix of tourists and protesters was a potentially complicated and dangerous one for police, who used tear gas to break up at least one demonstration on Saturday night.

"I was pretty nervous because I saw the riot police - never been in that situation in my life before so, I was kind of shocked," Raj Nadesan, who was in Montreal for the race told Reuters.

"This is the first time I've seen police presence (on the metro)."

On a quiet Sunday morning, there were few signs of protesters in Montreal's downtown core. One lone student stood at an intersection banging a saucepan, which has become a symbol of the student protest.

The VIP guests and sponsors were cloistered safely away inside the well-protected paddock area.

(Additional reporting by Leila Lemghalef in Montreal, editing by Alan Baldwin)