Dozens of civil rights protesters blocked the Metro Blue Line tracks during rush hour Tuesday as part of a nationwide demonstration against law enforcement treatment of people of color.
Police began making arrests after 5:30 p.m., broadcast news showed.
"We want to make sure nobody gets hurt and try and remove them peacefully, and if they don't listen we'll have to take whatever action we need to," Lopez said.
Images of demonstrations and arrests in the Bay Area, New York and dozens of other cities were shared under the social media hashtag #ShutDownA14 and #BlackLivesMatter.
The Los Angeles protesters gathered outside Los Angeles Police Department headquarters in the afternoon, then moved to the Blue Line tracks near Los Angeles Trade Technical College off Washington Boulevard. The trains were stopped after 4 p.m., before they could reach the 7th Street Metro station, a main hub for Blue Line commuters from as far south as Long Beach to connect to the rest of the rail system. The line serves about 80,000 commuters daily, officials said.
Metro officials said they had dispatched buses to help shuttle commuters to trains on either side of the blockage, but riders were delayed up to 20 minutes and agency officials could not say when service would be fully restored.
The demonstrations come on the heels of several controversial incidents between police and members of the public, particularly minorities.
On April 4, a police officer in South Carolina shot and killed a black man running away from him after a traffic stop. On March 1, LAPD officers fatally shot a homeless black man on skid row after the man allegedly grabbed an officer's holstered gun during a struggle, police said.
Last week, about 10 San Bernardino County sheriff's deputies repeatedly struck a man who had appeared to try and surrender after he mounted a horse and led them on a pursuit in Apple Valley.