A small group of criminals are to blame for the violence that happened Saturday before leaders of the G20 countries meet to discuss global economic recovery, said Toronto Mayor David Miller.

"I can only call them criminals and they should be arrested," the mayor told Xinhua on Saturday, after a 10,000-people protest in the downtown area turned into a riot.

“People are calling them protestors. That is not fair to the people who came to protest,” he said.

The riot left at least two arrested and three injured, only a few hours before the opening of the G20 summit.

The protestors also smashed shop windows in the downtown core, burned two police cars and tear gas was used by police at one site.

The mayor told media on Thursday that he was confident that the Toronto G20 summit would be a "safe" one. Canada has spent more than 1.2 billion Canadian dollars (1.15 billion U.S. dollars) on security of the summit.

What occurred Saturday has left many uncertainties about the securities of the summit.

"What's happening now at the G20 is not because of protestors -they are more like terrorists. It's a shame they are ruining Toronto streets," a Toronto citizen wrote on Twitter.

On Saturday after the riot the G20 security zone was put on lockdown as well as several other locations including the Eaton Center shopping mall, Sick Kids Hospital and Toronto General Hospital.

Two police cruisers were lit ablaze as flames and black smoke billowed into the air. Sparks flew into the air from small explosions. Firefighters arrived after about 15 minutes.

Tear gas was used to disperse the protestors. "I could hardly remember when it (tear gas) was used last time in Toronto," Laura Gao, a fast food shop owner told Xinhua. She witnessed the whole process.

A Xinhua photographer also witnessed a roadside Starbucks store smashed by some protesters. There were at least three Starbucks smashed and every single window of a major bank was poked a hole.