Egypt's al-Wafd Party demanded on Sunday that President Mohamed Mursi intervenes to put an end to the assault on the party's main headquarters in Cairo's Dokki.
"Islamists should know that al-Wafd Party is not weak and that its members can defend their party. Political Islam currents should understand that the party will stand up to them," al-Sayed al-Badawi, head of the party, said in a statement broadcast on TV.
"I urge the president to intervene immediately," prominent leading figure Fakhri Abdel Nour said.
The party said supporters of Hazem Salah Abu Ismail attacked the party's headquarters and set its newspaper headquartes alight.
A number of people were injured in the attack on the building, al-Wafd Facebook page reported.
According to the page, al-Wafd secretary in Alexandria said he received a phone call with threats to torch the party's headquarters in Alexandria.
The number of attackers was around 500, Tarek al-Tohami, member of the party's higher committee, said, adding that they were carrying automatic weapons and petrol bombs.