Hundreds turned out on Thursday in support of Egyptian President Mohamed Morsi's new declaration granting him sweeping powers, as a rival protest criticised "the making of new dictator."

"The people want the cleansing of the judiciary," chanted hundreds of Islamist protesters, who had gathered outside the High Court three hours before Morsi had even made his announcement.

The new declaration allows the president to "issue any decision or measure to protect the revolution," according to the text.

Morsi also ordered the reopening of investigations, and retrials, in cases of the killing of protesters during the uprising that ousted president Hosni Mubarak.

"I am here to celebrate the president's decisions," said Mahmud Abbas, 29. "I am really happy. The decisions are pro-revolution."

They "will bring justice to the martyrs and will hold the corrupt accountable," said Mahmud Sultan, among the crush of protesters in central Cairo.

But a few kilometres (miles) away in Mohammed Mahmud street, the scene of four days of clashes between police and protesters, there were vocal denunciations of Morsi's declaration.

"We didn't have an uprising so that we put in place another dictator," said a furious Khaled Ali.

"He's not just a pharaoh, he thinks he's God," said Ali.

"Justice will only come when Mohamed Morsi leaves," said Alaa Zaghloul, carrying a sign that read "Mohamed Morsi Mubarak."

The decision will once again pit Islamists against secularists and comes on the eve of planned protests in Tahrir Square to demand justice for hundreds who died during the 2011 uprising.

Morsi, who hails from the powerful Muslim Brotherhood, is the first elected president since the popular uprising that toppled Mubarak last year.