Unidentified "thugs" armed with guns or batons attacked demonstrators who included hundreds of ultraconservative Salafi Islamists protesting at the exclusion of their candidate from this month's vote, state news agency MENA reported.

The violence casts a deep shadow over the presidential election due on May 23 and 24, with a run-off in June, and highlights the fragility of Egypt's transition to democracy, which has been punctuated by violence and political bickering.

Security and medical sources gave a toll of 11 dead and over 160 wounded in the clashes outside the Defence Ministry in central Cairo's Abbasiya district. The fighting raged on unabated through the morning, but subsided in the afternoon.

"Where is the army? Why are they not stopping these people?" cried a bystander as the violence persisted. Shots rang out as young men ran back and forth across debris-scattered streets, hurling rocks, glass and petrol bombs.

"Down, down with military rule," yelled protesters.

Wounded men were hauled away as others filled bottles with petrol while shots rang out.

The army sent in extra vehicles and troops, but pledged in a statement not to disperse peaceful demonstrators.

Troops had earlier blocked a road to the ministry with army vehicles, barbed wire and troops in riot gear. Graffiti hostile to the ruling generals plastered walls on a main road nearby.

Days of street violence also preceded the start of a staggered parliamentary election in November.

The Muslim Brotherhood, which topped that poll, said it would boycott a meeting the army had called for Wednesday with political party leaders to defuse a crisis between the Islamist-dominated parliament and the army-backed interim government. (Reuters)