OAKLAND -- An Oakland elementary school was locked down and evacuated Monday afternoon after an anonymous caller reported that a man carrying a rifle had entered the campus, though no gunman was ever found.

"We found no evidence that the man was even on the campus, but we have to take every call like this very, very seriously," said Oakland police spokeswoman Holly Joshi.

Oakland schools police Chief Pete Sarna said it appears likely the call was a hoax and that it was placed from a school phone line.

If it was a prank, it was a costly one. Sarna estimated 75 officers from various agencies were dispatched to the scene, but he did manage to find a silver lining. The level of coordination between those agencies, he said, "was really remarkable to see."

Armed officers from several agencies spent all afternoon escorting hundreds of students from La Escuelita Elementary School at Third Avenue and East 12th Street after the caller, who said he was a custodian, used the police department's non-ergency number at 12:44 p.m. to say the gunman had entered a portable building on the campus.

Several streets around the campus were quickly blocked off as armed police methodically escorted the students to a building on campus, and then once they were all accounted for, to safety a few blocks away on East Eighth Street, said Joshi.

After the evacuation, police carried out a search of the campus, first with a robot, then a dog, then with SWAT officers, but did not find anyone. There were no injuries, and the scene was cleared at 4:50 p.m.

Hundreds of anxious parents clustered in a parking lot a few blocks from campus, alternatively talking on cell phones or to one another as a California Highway Patrol and news station helicopters buzzed overhead. Some were crying.

Heather Hall was visibly relieved when she finally had her son Dante, 5, in her arms.

"I've been mostly just worried about my son, and I'm very happy he's safe," she said.

Staff writers Cecily Burt and Ross Cameron contributed to this report.