“We are still investigating the cause of the explosion” which occurred late on Tuesday, police inspector Patrick Chademana told The Herald newspaper.
He said nobody was injured by the blast, which took place in the city of Gweru, 275 kilometres (170 miles) south of the capital Harare, and shattered the building’s windows.
Passmore Washaya, ZANU-PF’s provincial administrator, said the blast was “an act of aggression by our enemies, especially the MDC,” referring to the Movement for Democratic Change.
The MDC, which is led the country’s prime minister and Mugabe’s uneasy governing coalition partner Morgan Tsvangirai, denied the allegation as unfounded.
Mutual recrimination has intensified between the two parties in recent weeks and the country is expected to hold a fresh election in 2012.