Egyptian security forces shot dead at least 70 supporters of ousted President Mohammed Morsi early on Saturday, his Muslim Brotherhood said, deepening the turmoil which has convulsed Egypt for weeks.

Brotherhood spokesman Gehad El-Haddad said the shooting started shortly before pre-dawn morning prayers on the fringes of a round-the-clock vigil being staged by backers of Morsi, who was toppled by the army more than three weeks ago.

Al Jazeera's Egypt television station reported that 120 had been killed and some 4,500 injured in the early morning violence on the fringes of a round-the-clock vigil being staged by backers of Morsi near Cairo's Rabaa al-Adawia mosque.