MONTREAL - Ongoing student unrest has prompted warnings from Montreal's tourist industry and even a security message issued by the U.S. government.

American tourists were warned of possible "unforeseen violence,'' "vandalism'' and "arrests,'' in a message issued Apr. 27 by the U.S. embassy in Ottawa to visitors heading to Montreal.

The message warned Americans traveling to Montreal that although businesses remain open, traffic and public transportation may be disrupted.

Tourists are told they could run into demonstrations and, while most of them have been peaceful, some can turn "potentially violent.''

U.S. diplomats went out of their way Friday to downplay the significance of the memo; they called various media outlets to point out that it was to be referred to as a "security message'' and should in no way be confused with travel warnings like those issued to 31 countries including Syria, Haiti and Iran.

They billed it as a routine diplomatic message.

"When we issued this security message, there was an enhanced police presence and some of the demonstrations had been violent. Rocks were thrown and there were instances of vandalism,'' a consular official said.

Still, the news was another blow to many in Montreal's tourist industry, who fear that their livelihoods could be threatened by the ongoing tuition protests.

And it appears their concerns are warranted. Some tourists interviewed by CTV Montreal on Friday said they are wary of potential riots and violence.

"We've been very vigilant about what areas we've been visiting and have been ready to change our itinerary if need be," said Diana Candido, a school principal visiting with her students from Woodbridge, Ontario.

The summer's biggest event, the Formula One Grand Prix race rolls into town in less than a month and the tourist industry wants that to go without a hitch.

"We're very concerned," said Steve Siozios of the Crescent Street Merchants' Association. "We've been delaying making plans but we can't delay it anymore, we have meetings set with the police force."

"We have three four months that we all shoot for to make as much money as we possibly can."Crescent street merchants say business has gone down about 20 percent in the last three months already and they simply can't afford to take another hit.

One student association, comprising arts majors at UQAM university, decried this week that the protest movement had actually been too peaceful.

The association has a new objective, laid out in a motion adopted this week: to force the cancellation of the Grand Prix, an event its motion decried as a vessel for sexist, anti-environmental, elitist jet-setters.

Calculating the potential economic impact of ongoing unrest is no easy task.

According to the Board of Trade of Metropolitan Montreal, just the 90-minute delay caused Thursday by smoke bombs in several metro stations cost $11 million in lost productivity. The Montreal Economic Institute, a conservative think-tank, came up with its own figure of $9.3 million.

The disruptions have been so frequent, and so varied in size, that it's hard for economists to accurately pinpoint the economic fallout. Daily events have ranged from brief and peaceful street demonstrations, to riotous scenes, to this week's shutdown of metro service during rush hour.

The president of the Board of Trade of Metropolitan Montreal, Michel Leblanc, says Montrealers are now developing habits to avoid the downtown core because of fears originating from the recent protests.

He says people don't fear violence as much as they fear delays caused by the protests _ like being late for work or getting stuck in a traffic jam.

Candido, who had planned to return this summer on a personal visit, has changed her plans.

"I have family from Italy coming and we're going to avoid the city in the summer months and probability head to Mont Tremblant instead," she said.

with files from The Canadian Press