Hundreds of supporters of Democratic Republic of Congo opposition leader Jean-Pierre Bemba have staged angry protests in the capital, Kinshasa.
Mr Bemba's supporters accuse the president of dirty tricks
They set up burning barricades on one of the main streets and threw stones at UN peacekeepers, before police dispersed them by firing tear gas.
They blame allies of President Joseph Kabila for Monday's fire at their party base, which houses two TV stations.
Mr Bemba is to contest an election run-off against Mr Kabila next month.
The authorities say they have started an investigation into the fire but have not yet released their results.
But Vital Kamerhe, Secretary General of the ruling PPRD party rejected the accusation that his party was behind it.
"It is absolutely wrong. We have nothing to do with this. Bemba's people pretend to be the fire brigade while they are in fact the arsonists."
The BBC's Arnaud Zajtman in Kinshasa says the building was very old, as were its electrical installations.
The announcement of the first round results led to three days of gun battles in Kinshasa between Mr Bemba's and Mr Kabila's security forces, which left at least 23 people dead.
The second round will conclude the country's first democratic polls since independence in 1960.
Mr Bemba, a former rebel leader, came second in the first round, with 20% of the vote, compared to 45% for Mr Kabila.