San Francisco police spokesman Sgt. Neville Gittens said a man in his 30s walked into a law office on the 30th floor of a building at 275 Battery St. around 12:40 p.m.
The man had a white plastic device strapped around his waist and appeared to pose a threat, Gittens said.
Police were called and took the man into custody.
Floors 20-30 of the building were evacuated, and Battery Street was shut down while a police bomb squad was called in.
Gittens did not specify which firm was allegedly threatened but the law firm Lieff Cabraser Heimann & Bernstein, LLP is located on the 30th floor of that building.
As of 3:45 p.m. two San Francisco Police Department bomb squad trucks were still parked in front of the building, and the bomb squad remained inside, along with the suspect.
The device had been detached from the man and police were working to determine whether it contained any explosives.
Around 2:30 p.m., hundreds of employees dressed in business attire had formed a semicircle around the front entrance to the building, waiting to be let back in. Some chatted on their cell phones and others read the paper.
The crowd dispersed after a security guard announced that no one would be let back inside.
However, police have since been allowing people who work on the 20th floor or below to enter the building and collect their belongings. The other floors are still off-limits.
Gittens this afternoon did not know whether the man wearing the explosive device had any ties to Lieff Cabraser Heimann & Bernstein, LLP or what his motivation was for today's action.