SAN FRANCISCO (AP) — City police have joined a massive manhunt for the ‘‘night stalker’, now believed to have struck in San Fran cisco after committing a string of vicious murders in greater Los Angeles. Law officers from both cities ref used Thursday to reveal details that convinced them the ‘‘night stalker’’ had invaded San Francisco’s Lake Merced district over the weekend. “The whole department has been mobilized in our attempt to ap prehend the suspect,’ said Com mander Richard Klapp of the San Francisco Police Department The grisly murders and assaults started last March, and the killer is suspected of slaughtering at least seven people and assaulting as many as 13 others in the San Gabriel and San Fernando valleys. The victims were shot to death, stabbed or beaten. Some women were sexually assaulted by the killer, also dubbed the ‘‘valley intruder.”