Tens of thousands of protesters massed in central Cairo's Tahrir Square on Friday for one of the biggest demonstrations since the January uprising that brought down President Hosni Mubarak.

The protest sprang from the proposal of a "supraconstitutional principles" document proposed by the interim government, which answers to the Supreme Council of the Armed Forces (SCAF) that took power after Mubarak. The SCAF supports the document, which attempts to enshrine the military's role as the protector of the constitution and shield its budget from scrutiny.

Islamists such as the Salafi Nour Party and Muslim Brotherhood oppose the document, in large part because they believe it would push Egypt toward becoming a more secular state. Their organisational acumen brought out the crowds on Friday, but the demonstration represented a broad swath of parties and movements.

Al Jazeera's Jane Arraf reports from Cairo.