CAIRO, July 1 (Reuters) - The headquarters of Egypt's ruling Muslim Brotherhood was overrun by youths who ransacked the building after those inside were evacuated on Monday following a night of violence that killed at least eight people.

By far the bloodiest incident of Sunday's mammoth and mostly peaceful protests against the Brotherhood and President Mohamed Mursi, it began after dark and continued for hours, with guards inside firing on youths hurling fire bombs and rocks.

A spokesman for the Brotherhood blamed the violence on "thugs" and said it would be demanding answers from police who failed to protect it. He said two of those inside were injured - by fires - before a security detail from the movement was able to evacuate all those inside the compound in mid-morning.

The violence will likely add to a sense among Brotherhood members, long oppressed under Hosni Mubarak, that they face a political siege since being elected last year, which they blame on liberal opponents and loyalists of the old regime.

Images of the four-storey suburban building, its walls scorched, windows smashed and looters making off with office furniture, recalled those of the destruction of the state security headquarters when Mubarak was toppled in 2011.

The Brotherhood had fortified the headquarter's walls in the run up to the protests. The building was also attacked earlier this year in protests against Mursi.

Medical and security sources told Reuters that at least eight people had been killed in the violence at the Brotherhood headquarters - apparently all outside since the movement said its staff suffered only two wounded. State news agency MENA put the death toll in the incident at eight.

Medical sources said more than 100 people were wounded.

A security source put the total number of dead across the country since Sunday at 16, with 781 injured. (Reporting by Asma Alsharif and Tom Perry; Editing by Alastair Macdonald)