It was the former president's first public appearance since his wife, New York Sen. Hillary Clinton, ended her presidential campaign on June 7, after Obama emerged as the Democratic candidate in the November election.

The former first lady endorsed Obama, urged her supporters to rally behind him and is scheduled to campaign with him later this week.

But her husband has not publicly endorsed the presumptive Democratic nominee to succeed President George W. Bush. Asked by journalists when he might do so, Clinton smiled and shook hands with spectators without acknowledging he heard the question.

In his speech, Clinton predicted Congress would pass a cap-and-trade system to reduce carbon emissions.

"Because I believe so strongly in this, I favor Senator Obama's position, which is to go to 80 percent reduction in greenhouse gasses over Senator McCain's position, which is to go to 70 percent," he said. "But that's light years ahead of where Republican's have been."

Obama won heartier praise from Toronto Mayor David Miller, who said he was greatly impressed by the Illinois senator's speech to the mayors on Saturday.

"I wish a leader of one of Canada's federal parties would say the things he said," said Miller, who was invited to discuss climate change with the US mayors.

(Editing by David Wiessler)