Treasury Board President Stockwell Day is blaming a U.S. government travel alert issued for the city of Toronto on "a small group of thugs" who plan to protest as world leaders descend on Canada's largest city for the G20 summit next weekend.

On Thursday, the State Department issued the travel alert for U.S. citizens either travelling to or residing in the city, warning of "the potential for large-scale demonstrations" prior to and during the summit on June 26 and 27.

"Previous G-20 summits have drawn large numbers of protesters and activists, and a number of groups have announced plans to demonstrate throughout downtown Toronto," the alert says.

"Protesters are expected to gather in Toronto in the days leading up to the Summit and access to parts of the downtown area will be restricted due to security requirements. Even demonstrations that are meant to be peaceful can become violent and unpredictable. You should avoid them if at all possible."

The alert expires on June 28.

U.S. officials have downplayed the alert, saying it is standard procedure ahead of any international summit or similar event. But Day said it reflects fears of violence, particularly in the wake of the recent firebombing of a Royal Bank branch in Ottawa by a group that threatens violence during the G20 and preceding G8 summit in Huntsville, Ont.

"I think it shows the unfortunate power that a small group of thugs has," Day told CTV's Question Period on Sunday. "And when I'm talking about thugs I'm talking about thugs, I'm talking about the anarchists and the violent groups who have already indicated that they're going to be there and they're going to cause trouble."

Day said in the event of violent protests during which people get hurt, he hopes that the perpetrators are arrested and "hit with the heaviest sentences possible."

"I mean, we want to send a message that Canada continues to be one of the safest places in the world," Day said. "But it shows the effect when you get these thugs and cowards talking like this and threatening like this, other countries may feel that they should let their own people know about that."

The alert warns that the security fence built around the Metro Toronto Convention Centre, where the summit is taking place, will likely lead to "significant traffic disruption in the downtown core." It also predicts there will be public transit disruptions.

Toronto Police have said 3,500 officers will be devoted to G20 summit security, in addition to 1,600 officers from elsewhere in the GTA. The RCMP will also be providing security, while the Canadian Forces' JTF 2 unit will be on standby.

Toronto city councillor Adam Vaughan, whose ward includes the summit location, told Question Period the alert is "a little anxiety-driven and a bit of an overreaction."

Vaughan said the summit should be held at the isolated Canadian National Exhibition grounds, so it would have minimal impact on the city and its residents.