WASHINGTON - A crowd that organizers said numbered approximately 35,000 braved the cold on Sunday and marched to urge President Barack Obama to reject the Keystone XL pipeline and to show leadership on other climate issues they called urgent.

The group rallied on a slice of the Mall just north of the Washington Monument before heading down Constitution Avenue, up 17th Street and past the White House chanting slogans such as "We are unstoppable, another world is possible" and "Hey, hey, ho, ho, Keystone pipeline's got to go."

The president wasn't home, however. He was in Florida playing golf with Tiger Woods and Jim Crane, a Houston businessman who owns the Houston Astros as well as the residential compound where Obama is spending the holiday weekend.

But the demonstrators tried to send him a message nonetheless, carrying signs opposing not only the proposed pipeline from Canada to Texas, but also opposing hydraulic fracturing, or fracking, and coal plants. "Windmills, not oil spills," one placard said. Another said, "Fossil fuels? Fossil fools." And another: "Read my lips: no new carbons."

Leaders of the rally said they wanted to press Obama to follow up on the strong rhetoric in his inaugural address about the need to slow climate change. The official posters at the rally borrowed Obama's campaign slogan "forward." They read: "Mr. President, Forward - on Climate."

"Mr. President, we have heard what you've said on climate; we have loved a lot of what you've said on climate," said Michael Brune, executive director of the Sierra Club. "Our question is: What will you do?"

For many of the rally leaders, the first test will be whether the president and Secretary of State John Kerry approve a construction permit for the northern leg of the Keystone XL pipeline, which would carry crude from the oil or tar sands of Alberta to refineries on the Gulf Coast of Texas.

The energy-intensive methods needed to extract that crude emit more greenhouse gases than oil production methods from conventional reservoirs.

"Mr. President, you hold the pen and the executive power of hope in your hands," Brune said. "Take out that pen, Mr. President, and write down the words 'I reject the Keystone XL pipeline.' "

Sen. Sheldon Whitehouse, D-R.I., told members of the crowd that they could help encourage Obama. "We are going to have the president's back and he is going to have our back," Whitehouse said, adding that "We are going to look at our grandchildren and say 'Yes, we did.' "

But the rally had an edge of uncertainty about how hard Obama will push to take legislative or executive action. And most of the speakers zeroed in on the impending Keystone XL decision.

Those speakers included Bill McKibben, a Middlebury College professor who has led the fight to stop the pipeline; two leaders of First Nation tribes in Canada; and Tom Steyer, an investment fund manager in California and major fundraiser for Obama. All are strong foes of the Keystone project.

"If this pipeline goes through, your government will help in the raping and pillaging of the land of my ancestors," said Chrystal Lameman, a member of the Beaver Lake Cree Nation in Canada. "Then 1/8the companies 3/8 promise to give back what was never theirs in the first place."

Steyer said: "I get the argument for the Keystone. The argument is that it is business as usual because we use fossil fuels. But the time for business as usual has passed."

Afterward, Whitehouse said that "if the president and Secretary Kerry choose to approve the pipeline and proceed, there will be a massive credibility gap between that and what he said in the inauguration, especially if this is the first deed out of the box. That will be a problem for him."

Many of the demonstrators were college students who had traveled for hours by bus to take part in the rally.

Meghan Stratton, a student at the State University of New York at Binghamton, held a sign saying, "Tell Gov. Cuomo Don't Frack New York." She had traveled five hours by bus with other students.

"You can't ignore what's happening to our planet, and we want the president to know we're thinking about it," she said.

Ellie Whitney, a retired biologist active in a group called Citizens Climate Lobby, had traveled by bus from New Jersey and carried a sign that said, "Read my lips: no new carbons."

There was also a contingent from the Occupy Wall Street movement. They marched behind a long banner that read, "Occupy Wall Street, Stop Keystone." Some dressed as grim reapers, including one carrying a paper scythe with the words "tar sands."