But neither air travelers nor transit riders should encounter any new security procedures, officials said.
Muni and BART, the region's largest transit systems, are putting more officers on trains and in stations, and have been in contact with the Transportation Security Administration. No threats have been made against either agency, nor against U.S. transit systems in general, but many of the nation's large transit operators, including those in New York, Chicago, Los Angeles and Washington, are taking precautions.
Homeland Security Secretary Janet Napolitano said Monday that the agency would not issue an alert at this time. Abroad, U.S. embassies and other foreign facilities were placed on high alert, and strong admonitions were issued against U.S. citizens to be careful if traveling or living overseas.
BART canceled a training session for its Critical Asset Patrol Team of seven officers specially trained in counterterrorism Monday, and brought in extra officers to bolster its forces. The officers are walking trains and stations to provide a visible deterrent. The transit agency has also deployed its transportation supervisors, wearing neon green vests, to assist customers and provide extra eyes and ears.
"If you seen anything suspicious, please let us know," said spokesman Linton Johnson.
Sgt. Edgardo Alvarez, who oversees the special patrol team, said passengers should be on the lookout for abandoned luggage or packages and "people acting differently than the average commuter." That could include someone wearing overly bulky clothing in warm weather or with a tangle of wires under their jacket, he said.
Muni is also deploying extra officers throughout its system, spokesman Paul Rose said.
Both BART and Muni said the stepped-up patrols would continue indefinitely.
Other transit agencies, including Caltrain, the Capitol Corridor, the Altamont Commuter Express and AC Transit either declined to discuss security efforts or said they had not made any changes. All said they've been in contact with the TSA.
At San Francisco International Airport, spokesman Mike McCarron said air travelers won't notice any different procedures at security checkpoints, but will likely see additional police officers inside the terminals.