A suicide car bomber struck Cairo police headquarters, in the first of four blasts in Egypt’s capital that killed six people on the eve of the anniversary of the 2011 uprising.

Security forces and Islamist protesters have clashed around the country, leaving 51 killed, as a national holiday celebrating the military turned to mayhem.

Thirty eight people have been killed in clashes between security forces and supporters of the ousted Islamist president, Egypt’s health ministry said.

Egyptians were voting today in the second and final phase of a referendum on an Islamist-backed constitution, but there was little indication that the result of the vote will end the political crisis in which the country is mired.