He blamed the assault, captured by a TV crew, on the rightwing Sri Ram Sene even as one of the attackers was held.
The youths barged into his chamber and started beating him up, before being overpowered by lawyers and others present in the room, TV visuals showed. The youths objected to his comments on Kashmir, Bhushan said.
"They were Sri Ram Sene activists. They are intolerant," a visibly shaken Bhushan told IANS. He has lodged a first information report with police.
While one attacker was held, two others are said to have escaped.
Two weeks back, Bhushan had gone to Varanasi where during a press conference he was asked if he would favour a referendum in Kashmir. To this, he replied there could be a referendum, he said. It is this comment that angered the group, Bhushan said.
Many expressed shock over the incident.
"No words are enough to describe this barbaric act. Strictest punishment should be given to these people and the true identity of these people should be found," Congress spokesperson Abhishek Manu Singhvi said.
Team Anna member Kiran Bedi said: "They are attacking such a reasonable man who works for his country. The attack shows how intolerant society has become, how desperate we become if anybody has a dissenting voice."
The Sri Ram Sene had shot to notoriety in 2009 for attacking young couples in Karnataka and for opposing Valentine's Day on the ground that it was against Indian culture.