Protesters flocked to a district in downtown Cairo after the headquarters of Tamarod, a campaign against Egyptian President Mohamed Mursi, was set on fire early on Friday.
They chanted slogans against Mursi and the Muslim Brotherhood, the privately-owned Egyptian daily Youm 7 reported.
The group told Ahram Online that it had received “threats of physical harm from Islamist presidential supporters in recent days.”
Tamarod was initiated to gather 15 million signatures for a petition to withdraw confidence from Mursi.
Unknown assailants threw Molotov cocktails at the office on Friday, according to a statement posted on the campaign’s Facebook page.
The group wants to exceed the 13.2 million votes for Mursi during the presidential elections in 2012, which followed a revolution the previous year that toppled Hosni Mubarak.
Tamarod said it has 7 million signatures so far, the Associated Press reported.
Organizers said the petition forms were not severely affected by the fire, Ahram Online reported.
Attempts at intimidation “will only strengthen the determination to bring down the regime and press on with the revolution,” Ahram Online quoted campaign spokesman Hassan Shahin as saying.
The petition calls for early elections, but Mursi has insisted on completing his four-year term.
“We held a free and fair election, and to talk of an early presidential election is absurd and illegitimate,” he told Ahram.