The rally in view of the US Capitol was the culmination of a day of demonstrations around the world including in the Danish capital where tens of thousands took to the streets near the venue of the 194-nation summit.

Unlike in Copenhagen, where fringes turned violent, the Washington demonstration was a smaller and orderly affair in which dozens of activists held a candlelight vigil as the sun set on the National Mall.

The activist group Avaaz.org set up a wooden model of Noah's Ark with the inscription "Climate Plan B," a reference to the story in the monotheistic faiths of the vessel to save Earth's representatives from a great deluge.

"The ark is ultimately a symbol of hope," said the Reverend Derrick Harkins of Washington's 19th Street Baptist Church. "We as human beings have been given the tools to make sure we can make our way to the other side of a crisis."

The demonstrators urged President Barack Obama to support an ambitious and binding treaty in Copenhagen. Faced with conservative resistance to US action on climate change, Obama wants Copenhagen to consist of a set of voluntary pledges by nations rather than a document with legal obligations.

"I would hope that none of us would be satisfied with platitudes that may emanate from Copenhagen and then evaporate with the next news cycle," Harkins said.

"There's no person of faith who in good conscience should not see this as a crisis," he said.

Pressure groups called similar rallies across the United States.

The United States is the only industrial power to shun the current Kyoto Protocol, but Obama has pledged to work with other nations on drafting future action against climate change.

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