The group had protested, citing that a secret pact between Common Opposition Presidential candidate Maithripala Sirisena and UNP leader Ranil Wickremasinghe had taken place.

Eyewitness said police were unable to bring the situation under control.

However, supporters of the UNP who were inside the building retaliated in a well coordinated backlash that sent the attackers running for cover.

The attack by Weerawansa's supporters comes in the aftermath of former UNP general secretary Tissa Attanayake alleging that Common Opposition Presidential candidate Maithripala Sirisena was involved in the secret pact with Wickremasinghe.

The two political heavyweights have dismissed the allegation as a desperate ploy of a visibly losing rival candidate President Mahinda Rajapaksa at next month's election.

Wickremasinghe has already made a police complaint on this regard, citing that his as well as Maithripala's signature had been forged in preparing the fake document that claimed of a secret pact between the two.