Several hundred protesters have gathered outside the W Hotel in San Francisco's South of Market neighborhood, where President Obama is making an appearance at a campaign fundraiser.
Police have blocked off the area in front of the hotel, located at Third and Howard streets, and Third Street has been shut down to cars. The intersection is closed to pedestrians.
The crowd stretched for half a block in each direction away from the hotel.
Many were there to protest Keystone XL, TransCanada Corp.'s proposed 1,700-mile pipeline that would stretch from Canada to the Gulf Coast.
They held signs reading, "Stop Keystone XL" and "Another Voter Against the Pipeline."
There were also protesters from the Occupy SF movement carrying signs that read "We are the 99 Percent."
Others in the crowd were there to draw attention to the recent federal crackdown on medical marijuana providers. One woman carried a sign reading, "Cannabis created more jobs than you."
Dozens of police officers were standing in the intersection with their riot helmets at their sides. A bomb squad truck and a fire engine were parked nearby in front of the firehouse on Howard Street.
Air Force One touched down at San Francisco International Airport at 12:55 p.m., and the presidential motorcade arrived at the hotel at 1:30 p.m.
About 200 people were expected to attend the fundraiser, which was to include a performance by singer Jack Johnson, an Obama campaign official said. Tickets to the event started at $5,000.
The crowd outside the hotel was starting to thin out by about 2 p.m. as some people who had stopped by to check out the activity on their lunch breaks headed back to work.
Police said several SOMA streets will remain shut down until 4 p.m., including Third Street from Folsom Street to Mission Street; New Montgomery between Mission and Howard streets; Hawthorne Street between Howard and Folsom streets; and the intersection of Third and Howard streets.
Obama is scheduled to leave the Bay Area shortly after 3 p.m. today for Denver, and will head back to Washington, D.C., on Wednesday.
He flew to San Francisco today from Los Angeles, where he also had fundraisers planned. The president made a surprise visit to a Roscoe's House of Chicken & Waffles on Monday night, then taped a segment for The Tonight Show with Jay Leno this morning that will air tonight.