* President makes announcement before any deal with opposition
* EU sources say opponents seek changes but expected to sign
* Lawmakers trade punches in parliament
* Death toll in two days of violence reaches 77
* Ratings agency S&P downgrades Ukraine again (Adds Tusk quotes)
By Sabine Siebold and Natalia Zinets
KIEV, Feb 21 (Reuters) - Ukraine's President Viktor Yanukovich announced plans for early elections on Friday in a series of concessions to his pro-European opponents, but it was unclear whether they would accept an EU-mediated deal to end violence that has left dozens dead.
Russian-backed Yanukovich, under pressure to quit from mass demonstrations in central Kiev, promised a national unity government and constitutional change to reduce his powers, as well as the presidential polls.
He made the announcement in a statement on the presidential website without waiting for a signed agreement with opposition leaders after at least 77 people were killed in the worst violence since Ukraine became independent 22 years ago.
"There are no steps that we should not take to restore peace in Ukraine," he said. "I announce that I am initiating early elections."
Yanukovich said Ukraine, which emerged from the wreckage of the Soviet Union in 1991, would revert to a previous constitution under which the president had less authority.
"I am also starting the process of a return to the 2004 constitution with a rebalancing of powers towards a parliamentary republic," he said. "I call for the start of procedures for forming a government of national unity."
Polish Prime Minister Donald Tusk, whose foreign minister is part of a European Union team trying to broker a compromise, said he could not be certain that the "worst-case scenario" could be avoided. "The threats are still there," he told a news conference in Warsaw.
The EU mediators said the opposition was seeking last minute changes, but they still expected a deal to be signed on Friday. There were fist fights in parliament as the political tension mounted.
The sprawling nation of 46 million with a shattered economy and endemic corruption is at the centre of a geopolitical tug-of-war between Russia and the European Union.
The German and Polish foreign ministers were in Kiev to promote a political compromise to end the bloodshed amid a stand-off between riot police and anti-government protesters who have occupied a central square for nearly three months.
Poland's Radoslaw Sikorski said he and Germany's Frank-Walter Steinmeier were going to meet representatives of the street protesters to discuss the draft agreement.
Ukraine was at a "delicate moment", Sikorski said on his Twitter account, adding in an apparent message to opposition leaders: "All sides need to remember that compromise means getting less than 100 percent."
A table was set up for a signing ceremony in the presidency building with nameplates for three opposition leaders.
Whether grassroots activists who want Yanukovich out now will accept such a gradual transition was uncertain.
"This is just another piece of paper. We will not leave the barricades until Yanukovich steps down. That's all people want," said Anton Solovyov, 28, an IT worker protesting in the central Independence Square.
Earlier, police said in a statement that anti-government militants fired on security forces near the square, scene of a three-month-old protest vigil. However, there was no confirmation of such an incident and no report of casualties.
The square, known as Maidan or "Euro-Maidan", appeared peaceful, with thousands of demonstrators chanting anti-government slogans interspersed with patriotic singing.
SCUFFLES IN PARLIAMENT
Armed police briefly entered the parliament building while lawmakers were holding an emergency session but they were quickly ejected, opposition leader Arseny Yatsenyuk said.
Ukraine faces the risk of civil war or even a break-up, and rage has spread even into the parliamentary chamber. Members exchanged punches when speaker Volodymyr Rybak tried to adjourn proceedings.
Opposition deputies were angered because it would mean delaying a possible vote on a resolution pressing for constitutional changes to restrict the president's powers. The speaker left the chamber and debate continued.
If signed and implemented, the deal would be a setback for Russian President Vladimir Putin, who has made tying Ukraine into a Moscow-led Eurasian Union a cornerstone of his efforts to reunite as much as possible of the former Soviet Union.
Putin appointed his own envoy to the talks at Yanukovich's request on Thursday but it was not clear what role, if any, Russian officials had in the negotiations.
YANUKOVICH SUPPORT EBBS
After 48 hours in which the fate of Ukraine was fought out in the square, Yanukovich was rapidly losing support.
The deputy chief of the armed forces resigned and opposition deputies in parliament voted to overturn severe anti-terrorist laws enacted by Yanukovich's government this month and ordered security forces back to barracks.
In another sign of the severity of the crisis, ratings agency Standard & Poor's cut Ukraine's credit rating for the second time in three weeks on Friday, citing the increased risk of default.
S&P said latest developments in the crisis made it less likely that Ukraine would receive desperately needed Russian aid. Ukraine cancelled a planned issue of 5-year Eurobonds worth $2 billion, it told the Irish Stock Exchange where the debt would have been listed. Kiev had hoped Russia would buy the bonds to help it stave off bankruptcy.
Russia's economy minister said Moscow was still undecided on the next $2 billion instalment and was awaiting clarity on the government in Ukraine.
The health ministry said 77 people had been killed since Tuesday afternoon, which meant at least 47 died in Thursday's clashes.
On Thursday, EU foreign ministers meeting in Brussels imposed targeted sanctions on Ukraine and threatened more if the authorities failed to restore calm. (Additional reporting by Richard Balmforth, Alessandra Prentice, Vasily Fedosenko and Pavel Polityuk in Kiev, John Irish in Paris, Marcin Goettig and Adrian Krajewski in Warsaw and Francesco Guarascio and Adrian Croft in Brussels; Writing by Paul Taylor; Editing by David Stamp)
Our Standards: The Thomson Reuters Trust Principles.