Egypt's supreme constitutional court has angered the Muslim Brotherhood by allowing a former Hosni Mubarak confidante to stand in the presidential elections.
The top Egyptian court ruled that Mubarak's ex-aide and former prime minister Ahmed Shafiq would be allowed to take part in the presidential race run-off.
The court ruled against a law which would have otherwise thrown him out of the race.
The Muslim Brotherhood, whose own candidate, Mohammed Mursi, is the only other candidate, described the decision as dangerous.
The court's decision sparked protests outside the court from members of the Brotherhood.
The party warned against power returning to those linked to the previous regime, said the BBC.
"All the democratic gains of the revolution could be wiped out and overturned with the handing of power to one of the symbols of the previous era," it said.
Mursi said: "If there is any forgery, there will be a huge revolution against the criminals, a huge revolution until we realise the complete goals of the 25 January Revolution."
He promised to stay in the fight for the run-off.
Shafiq supporters reacted happily to the ruling, clapping and dancing in the streets of Cairo.
"The message of this historic verdict is that the era of political score-settling has ended," an ecstatic Shafiq told reporters.
"Shafiq is a respectable man. We want him as president because we are not ready for Egypt to return to the Dark Ages," Reuters quoted a supporter.