A police spokesman said the raid on the building north of Athens "went smoothly" and that police removed about 50 former employees, The New York Times reported.
Officers fired tear gas to disperse about 200 protesters outside the building, but demonstrators returned after the police left and resumed their protest, witnesses said.
Excessive spending was the main reason authorities gave when they closed ERT in June, laying off more than 2,600 workers. Since then, former workers have occupied ERT's headquarters, airing a pirate Internet broadcast.
A transitional state broadcaster began operations in July and a successor to ERT is expected to start broadcasting next spring.
Government spokesman Simos Kedikoglou said Thursday's raid was to "apply the law and restore legality."
The crackdown came as Greek officials negotiating with representatives of the country's international creditors about a projected gap in Greece's 2014 budget and economic reforms, the Times said.
Greek political opposition, with around 10 members of Parliament from the leftist party Syriza, went to the scene when news of the raid became known, the Times said. Speaking outside the headquarters, Syriza lawmaker Zoe Konstantopoulou accused the government of "reaching the point of delirium" and "staging a coup against itself."
Giorgos Kogiannis, a former ERT news editor who was among the protesters, said at least one employee remained inside.
"It's a big building, they didn't find everyone," he said.