With thousands of people expected to fill the streets of Los Angeles for CicLAvia and other events this weekend, law enforcement officials say they are stepping up security because of the Boston bombings, even in the absence of a credible threat in Los Angeles.

Officials with the Los Angeles Police and Sheriff's departments say they are taking precautionary measures and urging the public to be alert for anything suspicious.

"We're concerned until whoever was behind the attack in Boston is taken into custody and we make sure they were acting alone and there's no nexus to Los Angeles," LAPD spokesman Cmdr. Andrew Smith said. "But we, as a country, need to maintain our vigilance forever. We're always concerned about copycats and whenever we have large gatherings of people. "

Among the events taking place over the weekend are a Bon Jovi concert at Staples Center, the Los Angeles Times Festival of Books at USC, and the Long Beach Grand Prix.

By far the largest public event is CicLAvia To The Sea on Sunday, when 15 miles of streets stretching from downtown Los Angeles' El Pueblo historic center to the Venice Beach boardwalk will be closed to all vehicles and taken over by people on foot, bicycles, scooters, skates and skateboards.

An estimated 100,000 people are expected.

Smith said the LAPD has looked at all large events as a potential target, even before the bombing in Boston.

"We certainly don't expect that to happen here but, like LAPD Chief Charlie Beck says, 'We're not in the optimism business,'" he said.

"Depending on the event and the venue, we'll have additional police officers in uniform and undercover, as well as bomb detection canine officers to sweep different areas and ensure there are no explosive devices," he added. "But we rely not just on the LAPD's 10,000 officers but the 4 million people in Los Angeles to let us know if there's something wrong, so if you see something, say something. "

With throngs of people expected to take buses and trains to the events, Lt. John Sullivan with the Sheriff's Department's Transit Services Bureau said security will be extra tight at Metropolitan Transportation Authority facilities and vehicles.

"With the tragic events in Boston this week, we have a stark reminder that terrorism is an ever-present threat," he said. "In response, our bureau has increased high-visibility patrols so you'll see deputies armed with assault weapons walking the terminals at Union Station and other locations. "

"We also have additional mobile search and screening teams to do random checks of bags and make sure somebody isn't carrying a bomb, as well as explosive detection canines to sniff the presence of explosive precursors and agents," Sullivan added. "We've briefed all deputy sheriffs and security personnel as well as all Metro employees to be aware of their surroundings. "

Sullivan urged the public to report suspicious packages left unattended, particularly those having wires or looking like they contain oil or water, as well as people who seem armed or look like they may be conducting reconnaissance, including taking pictures of heating and ventilation ducts.

"I could give you a specific list of suspicious activity but, in reality, we know what's out of place - often it gives you a funny feeling in the back of your neck," he said. "If it doesn't seem right, don't try to second guess it and don't worry about inconveniencing the police - we're here to be the public steward of safety. "

Los Angeles has hosted five previous CicLAvia events. They are based on a concept developed in Colombia several decades ago aimed at getting people out of their cars and walking or biking on the streets.

"CicLAvia provides a new way for people to interact and connect with Los Angeles, allowing them to experience parts of the city they may not normally visit and interact with different businesses they might not have known," CicLAvia spokesman Robert Gard said. "Most importantly, they can meet their fellow Angelenos in a more relaxed and enjoyable atmosphere compared to the typical journey to the city in a car. "

LA.StreetsBlog.org editor Damien Newton said he and his family will take bicycles to Culver City during CicLAvia "and ride around until the kids get tired. "

"What happened in Boston actually makes something like CicLAvia even more important because attacks like that are designed to make people afraid, keep them inside and separated," Newton said. "The whole purpose of CicLAvia isn't just the party and the fun - it's to help bring Los Angeles together. "

Metro riders who see a suspicious package or witness a crime should call 911 or 888-950-SAFE (7233)

The Sheriff's Department has launched the website transitwatchla.org with up-to-date information on safety and security enhancements at local public transit systems.