City streets around the convention centre will be sealed off for next month's G20 summit in Toronto and much of the downtown will experience "significant traffic disruptions," police announced Friday.

Toronto police Superintendent Tom Russell said construction work will begin next week on a security fence that will surround the blocks near the Metro Toronto Convention Centre, where the summit is being held.

"As with most international summits, security fencing will be used to secure the area," Russell said. "We know this will be inconvenient to Torontonians and we ask for your patience and understanding."

The fenced-in zone will extend around the Toronto Convention Centre, the site of the two-day meeting of the leaders of the world's most prosperous nations, but will also encompass some of the city's busiest streets, biggest hotels as well as the CN Tower.

"These boundaries are still subject to change," Russell said. "In fact, we are currently under negotiations with business partners including the Rogers Centre within this perimeter."

Only authorized cars -- such as police, army or emergency vehicles -- will be allowed inside the secure zone and Toronto's already slow-moving downtown traffic will be restricted several blocks away from the site of the summit.

The downtown headquarters of Canada's biggest banks are not part of the secure zone, although they will be within the larger, traffic diversion area, designed to steer traffic away from the site of the meeting.

Police say people will be able to move freely within the security zone until the day before the summit starts on June 26.

Russell said people living or working inside the security zone, which includes several condominiums and downtown office towers, will be allowed to come and go provided they have identification, but warned: "This process may take some time."

The announcement follows a vandalism spree downtown in which anti-G20 slogans were spray-painted overnight at several downtown Toronto banks. The banks and several automated tellers were spray-painted with slogans such as "Stop G20" and "Resist G20."

One man faces mischief charges and police believe another suspect also helped paint the slogans.

Russell said he believes most protests will be peaceful. "We believe that most groups will want to express themselves in a peaceful and responsible manner. However we are preparing for any eventuality."

Russell said the main highways near the summit site will not be closed down, although several of the off-ramps nearby will be closed off. "However there will be intermittent restrictions on Highway 427, and the Gardiner Expressway to accommodate motorcade travel (to the summit)."

The summit, which is to follow immediately after the G8 summit in Huntsville, Ont., north of Toronto, will require thousands of police officers and soldiers to provide security.

The bill for both summits is forecast to exceed $1 billion, drawing angry demands from the opposition for audits and charges of "reckless spending" by the government.

Bill Majcher, a security expert and former RCMP inspector, told CTV News Channel that setting the summit in downtown Toronto was likely the main reason for the high costs of security.

"Every time you have it in these large urban centres you have a gathering point for a lot of anarchists or people just looking to cause trouble. You have a security nightmare," he said. "But the police can only respond to what the government dictates and that's where these things tend to spin out of control in terms of cost and manpower."

He said violent, anti-globalization protesters are only one threat the combined police and military security unit must guard against. The possibility of a terrorist attack can never be ruled out, he said.

"The one thing that bothers me about this is that we're spending close to $1 billion just five months after we spent a similar amount in the Olympics for security … it's hard not to say that the terrorists haven't won this battle in Canada," Majcher said.

"Terrorism is all about fear, changing the behaviours of people based on fear and with these kinds of costs, from my point of view it seems like the terrorists may be winning."

Parliamentary Budget Officer Kevin Page has said his office will investigate the growing cost of security for the two summits.

Russell said holding both the G8 and G20 summits back-to-back created an organizational headache for security planners. "Never have the two summits been held in one weekend. This has presented unique challenges," he said.

"These summits will be put Canada on the world stage for three days in June and we will endeavour to ensure that security will not be the overarching theme."