WASHINGTON - President Barack Obama vowed Sunday, in a videotaped message played to tens of thousands at an immigrants' rights rally on the Mall, that he'd work with Congress to move forward on new immigration laws this year.

"I pledge to do everything in my power to forge a bipartisan consensus this year on this important issue," Obama said. "You know as well as I do that this won't be easy, and it won't happen overnight."

That vow prompted many on the crowded Mall, including the dozens who boarded five buses on Long Island early Sunday, to cheer with what one of them said later was a wary determination.

"It's good that we have his attention," said Luis Valenzuela, director of the Long Island Immigrants Alliance, of Obama as well as top lawmakers.

"But now we want to know that they heard us," Valenzuela said. "They need to demonstrate in actions, not words."

That sentiment echoed the words of Gustavo Torres of the immigrants' group Casa de Maryland, whose speech followed Obama's video.

"We are going to hold you accountable, President Obama," he said to applause.

Valenzuela and other immigrant advocates said they knew they were pressing their demands in the shadow of the biggest political drama in town: a down-to-the-wire House vote on Obama's top priority, historic health care legislation.

And they acknowledged that the chances of passing an immigration overhaul this year look bleak to many, with Washington divided by partisan rancor and lawmakers skittish as the fall's elections near.

Yet Pat Young, leading a contingent from the Central American Refugee Center in Hempstead, said the immigrants' rights movement has grown wiser since its 2006 march.

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"What you're seeing today is a mature movement," Young said. "It's not just in the streets like you see today, but it's also at the ballot box."

Valenzuela and Young credited Sunday's march with pushing Obama to hold a day of meetings on immigration, and Sen. Charles Schumer (D-N.Y.) and Sen. Lindsey Graham (R-S.C.) to finally release a long-awaited framework of their immigration legislation.

Organizers claimed the rally drew 200,000 people, twice as many as predicted. New York sent 227 buses carrying about 11,000 people, said Norman Eng of the New York Immigration Coalition.

Among them was Ivan Sacor, 28, who said he does construction work and landscaping in Riverhead, Southampton and Greenport. He said he wants to bring his wife and two children up from Guatemala, but under current law cannot.

Also attending was Joselo Lucero, whose brother Marcelo was stabbed to death in Patchogue in 2008.

At the rally, TV broadcaster Geraldo Rivera highlighted that killing and blamed radio and TV shows that spread bigotry against immigrants for influencing the teens accused of the murder. "They stabbed him to death because he was an immigrant and a Latino," Rivera said. "We have to ensure that immigrants cannot be slandered like this again."