Environmental activists encircled the White House yesterday -- sometimes standing four- and five-deep -- and in terms both political and starkly personal urged President Obama to reject the proposed 1,800-mile Keystone XL pipeline.

The Associated Press estimated the crowd at 8,000, but organizers said it was larger and represented a watershed moment in the fight against the pipeline, which as proposed would bring crude oil from oil sands in Alberta, Canada, to refineries in the Gulf of Mexico.

"It's a completely amazing sight," said Bill McKibben, the climate activist who served as emcee of the rallies that took place in Lafayette Park before and after environmentalists joined hands to surround the White House.

While the event drew protesters of all ages and from all walks of life -- and speakers at the rallies represented an array of interests -- the recurring theme was that Obama needs to show stewardship for the environment by rejecting the proposal and that his decision will have a great bearing on whether many of these activists are prepared to support him again. Scores of anti-pipeline signs were designed to look like Obama campaign posters, and they featured quotes from the president decrying climate change and the influence of big-monied interests on the political process.

"Our message to the president is simple: We are your base," said Heather Mizeur, a Maryland state legislator who also serves as an Obama appointee to the executive committee of the Democratic National Committee. "We are the Democrats who put you in office."

The Rev. Lennox Yearwood Jr., head of the Hip Hop Caucus, compared the Keystone XL battle to other history-changing protest movements.

"This is our lunch counter moment for the 21st century," he said. "President Obama -- do the right thing!"

Several speakers sought to dismiss one of the chief arguments for the pipeline, which is that it will create tens of thousands of jobs, by questioning studies on the project and job creation. While some labor unions, including the International Brotherhood of Teamsters, have embraced the project, others are opposing it, and Roger Toussaint, president of the Transit Workers Union of America, offered a full-throated condemnation.

"My check/President Obama," he chanted. "We want jobs, not lies/President Obama, we want jobs, but not jobs as gravediggers of the planet/Stop the pipeline!"

In addition to the rhetoric, the afternoon was full of stagecraft -- most notably a 180-foot-long plastic contraption meant to look like an oil pipeline, which dozens of protesters carried repeatedly around the White House and the streets of downtown Washington.

McKibben said the Keystone XL issue has united the various factions of the environmental movement like no other, and he told the crowd that "by virtue of you being here, you're now the leaders of this fight, and we need to get you out into all corners of this country."

The State Department has a tentative mandate to rule on the project by the end of the year. But it is not clear if that timetable will be met, and in an interview with a Nebraska TV station last week, Obama suggested that he might make the decision himself.

But despite the rally's general good cheer, the challenges the anti-pipeline movement face were never far away. Looming over Lafayette Park, with its hundred-foot-high banners calling for jobs and touting the website freenterprise.com, was the marble headquarters of the U.S. Chamber of Commerce.

Obama himself was golfing in Virginia while the protest was taking place.

And as protesters were shouting about the tar sands, one passer-by wondered, "Why do they want to stop Tarzan?"