(Adds final number of arrests, injuries)

By Louise Egan

MONTEBELLO, Quebec, Aug 20 (Reuters) - Protesters and riot police clashed on Monday outside a posh Canadian resort where North American leaders are meeting to discuss trade, security, and the recent turmoil in global credit markets.

Police fired tear gas as they pushed back about 150 people outside the Chateau Montebello resort in Montebello, Quebec, about 40 miles (65 km) east of Ottawa. The demonstrators tossed rocks, bottles and vegetables, having tried earlier in the day to break through police lines.

Four people were arrested, police said. Several police officers suffered minor injuries.

The protesters were among about 2,000 people who demonstrated for several hours outside the site of the meeting between U.S. President George W. Bush, Canadian Prime Minister Stephen Harper and Mexican President Felipe Calderon.

The leaders downplayed the demonstration. "I've heard it's nothing. ... A couple hundred? It's sad," Harper said to reporters before going into a meeting with Bush.

A 10-foot (3-metre) high fence had been erected around the site to keep demonstrators out.

The protesters represented a variety of political causes, but many are upset about the leaders' plan to discuss what is known as the Security and Prosperity Partnership, or SPP.

The plan was drafted in 2005 to ensure North Americah is a safe place to live and do business, but critics on the left and right say it ignores the concerns of ordinary citizens and is a threat to national sovereignty.

"One of the main things we are aiming for is to force the leaders to make these meetings more transparent," said Alberto Arroyo, a spokesman for the Red Mexicana de Accion Frente al Libre Comercio (Mexican network against free trade).

Arroyo was one of about a hundred activists who traveled from Mexico to the demonstration. Most others appeared to be from Canada.

"As far as I'm concerned George Bush is a war criminal and shouldn't be allowed in Canada," said Rolf Gerstenberger, 57, a union activist from Hamilton, Ontario.

One protest sign read: "There are no smart borders only idiot nations."

The buildings where leaders were meeting are not visible to the protesters, but those at the resort will be able to view the demonstrations via a video link that will be broadcast to officials inside the compound.

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