A febrile and polarised Egypt turned on itself on Friday as protests spread across the country, pitting supporters of the Islamist president, Mohamed Morsi, against his political opponents over a controversial new decree granting him extensive new powers.

Anti-Morsi demonstrators, who accuse the president of having launched a "constitutional coup" on Thursday, were reported to have set fire to the offices of the Muslim Brotherhood's Freedom and Justice party, to which Morsi belongs, in the Suez Canal cities of Suez and Port Said.

Clashes also erupted between the two sides in the Mediterranean city of Alexandria, the southern city of Assiut and Giza, prompting Essam el-Erian, a leading figure in the FJP, to condemn the attacks as "acts of thuggery hiding behind political forces".

In Cairo, the two opposing camps gathered in large rival rallies in different parts of the capital. In a packed Tahrir Square, youths opposed to the decree fought intermittent battles with police firing volleys of teargas outside the French Lycée and American University. Morsi's Muslim Brotherhood meanwhile bussed in supporters from across the country to hear him address a rally outside the presidential palace in Heliopolis.