The name board and glass windows of the party office were damaged in the attack, police said, adding some BJP workers inside the building escaped unhurt.
The Youth Congress activists allegedly also hurled tomatoes, eggs and stones at the office, police said.
The activists fled the scene before police arrived on the spot.
A BJP spokesman said they had registered a complaint with police in this regard.
Singh had a fews days back wondered if Congress would have accepted Sonia Gandhi's leadership if she was not white-skinned.