Egypt's opposition plans new protests against a draft constitution that looks set to be approved in the second round of a referendum next weekend.
President Mohammed Mursi obtained a 57% Yes vote in initial voting, less than he had hoped for.
The result is likely to embolden the opposition, which says the law is too Islamist.
However, the second round is expected to result in another Yes, while underlining the deep divisions that have riven Egypt since Hosni Mubarak's fall.
Yesterday, protesters broke out into cheers when the public prosecutor Mr Mursi appointed just last month announced his resignation.
They said it was a victory for the independence of the judiciary.
But they are unlikely to win Saturday's referendum second round, to be held in districts seen as even more sympathetic towards Mr Mursi's Muslim Brotherhood, which won elections held after Mubarak was ousted in February 2011.
The opposition National Salvation Front said there were widespread voting violations in the first round of the referendum vote and urged organisers to ensure that the second round was properly supervised.
It has called for protests across Egypt "to stop forgery and bring down the invalid draft constitution" and wants organisers to re-run the first round of voting.
In Cairo, the Front plans to hold demonstrations at Tahrir Square, cradle of the revolution that toppled Mubarak, and outside Mr Mursi's presidential palace, still ringed with tanks after earlier protests.
The build-up to the first round saw clashes between supporters and opponents of Mr Mursi in which eight people died.