By Tom Perry and Edmund Blair

CAIRO (Reuters) -- Activists called for a protest on Friday and Islamists warned that the gains of the revolt that toppled Hosni Mubarak could be wiped out after Egypt's supreme court dissolved parliament and ruled to keep his last premier in this weekend's presidential race.

Egypt's already troubled transition was plunged into turmoil by Thursday's court rulings. Islamists, who dominate parliament and have gained most since the uprising that ousted Mubarak last year, called the court ruling a "coup".

The run-off election on Saturday and Sunday was billed as the culmination of a transition before the military generals who have ruled Egypt for 16 months since Mubarak's overthrow formally hand power to a new president.

The new leader will now be elected without a parliament, whose election has been one of the few substantive gains, and without a new constitution to outline the extent of his permanent powers, a process delayed by political bickering.

Officials campaigning in the race ends a midday on Friday.

The April 6 movement, which helped galvanize Egyptians against Mubarak, called for a protest march on Friday that would head to Cairo's Tahrir Square "against the soft military coup." It called for the protest to start at 5 p.m.

"We will save our revolution. We will save Egypt from military rule," the group said in a statement sent out early in the morning on Friday.

The main target of the group's opposition is presidential contender Ahmed Shafik, a former air force commander who was appointed prime minister in Mubarak's last days in office.

They fear he will seek to rebuild Mubarak's repressive state and reverse the gains of the revolt, although he denies this.

He is pitted in the race against Mohamed Morsy of the Muslim Brotherhood, whose group secured the biggest bloc in parliament and with other Islamists forms a majority in the assembly.

US Secretary of State Hillary Clinton said the United States, long wary of Islamists and which provides $1.3 billion a year in military aid to Egypt, expected the military to fully transfer power to a democratically elected civilian government.

The Brotherhood said on Thursday that the court rulings indicated Egypt was heading into "very difficult days that might be more dangerous than the last days of Mubarak's rule".

"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.

Warning against forgery

Morsy pledged to press ahead with his presidential bid regardless and warned against foul play of the type that was typical of elections in Mubarak's days.

"If there is any forgery, there will be a huge revolution against the criminals ... a huge revolution until we realize the complete goals of the Jan. 25 revolution," he said, referring to start of the uprising against Mubarak on Feb. 11, 2011.

Shafik told his jubilant supporters at a rally at a hotel on the outskirts of Cairo that the court rulings were "historic" and said the "era of political score settling has ended."

"The army and the people are on one hand," his backers chanted.

Outside the court, protesters chanted "Down, down with military rule" and threw stones at troops lined up around on the building on the banks of the Nile. A few hundred demonstrators also gathered in Tahrir, though nothing like the hundreds of thousands that packed the square during and after Mubarak fell.

"Shafik is from the old regime. We can't have him back. We need to give Morsy a chance. The court rulings were wrong," said Salah Sayed Mahmoud, a 43-year-old carpenter who had joined those in the square on Thursday evening.

Shafik's backers include Egyptians who may have been happy to see the back of Mubarak but are tired of turmoil and see Shafik having the military experience to restore order. Morsy has the broad grass-roots network of his 84-year-old group.

Yet for many Egyptians, who voted for centrist candidates in the first round, the choice could not be worse. They worry about handing power back to a military man like Mubarak as much as about giving it to a conservative Islamist.

For them, the vote has reproduced the same battle lines that dominated Egypt's stale politics for years, pitting a man who like every president for six decades came from military ranks against their perennial antagonist, the Brotherhood.

"We are back to the political dynamic of secular versus Islamist, of a civil state versus an Islamist state," said Mona Makram Ebeid, a political scientist and member of a body that advises the military council, speaking before the rulings.

"That is what we as political forces are confronted with today, causing almost a gridlock," she said, referring to the months of wrangling between the army, Islamists, liberals and other parties seeking to carve a new course for the nation.