Thugs on Sunday set on fire the Southern Region Headquarters of the Malawi Congress Party (MCP) in Blantyre.
The party said the criminals also looted the offices.
MCP regional secretary for the south Peter Khofi told the local media that the thugs arrived at the offices in two minibuses and one pick up at around 10pm.
Guards at the premises fled upon seeing the thugs who then torched the building and stole items.
MCP Vice President Sidik Mia is expected to visit the offices today.
The incident comes days after the house of a Malawi Congress Party legislator and a UTM vehicle were also petrol bombed.
MCP and UTM are challenging results of the May 21 elections in which President Peter Mutharika of the Democratic Progressive was declared winner.
The two opposition have also been supporting protests against Malawi Electoral Commission chairperson Jane Ansah.