(Updates with latest figures)

By Paul Hoskins

BELFAST, March 8 (Reuters) - Northern Ireland's biggest Protestant and Catholic parties were far ahead of rivals on Thursday in early results from assembly elections, as the province took a step towards restoring shared government.

A strong showing for the main groups could make agreement on a power-sharing government more likely, sidelining the hardest liners from both communities who oppose a deal and moderates whose own attempt at joint rule collapsed five years ago.

With 50 of 108 seats decided, 20 went to firebrand preacher Ian Paisley's Democratic Unionist Party, the biggest pro-British-rule Protestant faction.

Twenty also went to Sinn Fein, the main Catholic party allied to the Irish Republican Army (IRA) which wants a united Ireland and fought against British rule for three decades.

Three other parties won 10 seats between them. Complete results are expected on Friday.

"I trust that before long, we will have a government in this country that is pledged and chained and tied to democracy," Paisley, who regained his own seat, told reporters.

Although Paisley has not said he is ready to talk to Sinn Fein, he has left the door open, dismaying some former followers who accuse him of betrayal.

"I will not be talking to Sinn Fein until they repudiate their terrorism. Let them foreswear their violence, let them turn a new leaf," Paisley said in his Ballymena heartland.

Sinn Fein leader Gerry Adams also won his West Belfast seat comfortably.

"The people have spoken and they have said 'get on with it' and that's what Ian Paisley has to do," he said.

BRITISH THREAT

Britain has threatened to impose indefinite direct rule, with help from Dublin, if Northern Ireland's parties do not meet a deadline of March 26 for agreeing on a government.

Attempts at transferring powers to Belfast have foundered repeatedly since a peace deal in 1998 largely ended a conflict in which 3,600 people were killed. The last assembly did not even sit for a full day after it was elected.

Agreement would suit British Prime Minister Tony Blair, who wants a political settlement before he steps down this year, and Irish Prime Minister Bertie Ahern, who plans to call a parliamentary election this year.

The two leaders have worked on a political solution for Northern Ireland for almost a decade and are due to meet in Brussels on Friday. Paisley said he expected to hold talks with Blair next week.

Ahern welcomed the good showing by parties seen as ready to accept power-sharing.

"I think their position has been vindicated, so that's good," he said in Brussels, also pointing to a solid turnout.

The share of the vote is not expected to change much from British parliamentary elections in 2005 when the DUP scored 34 percent and Sinn Fein 24 percent.

But both main parties face dissidents within their own constituencies who accuse them of betraying their principles.

Despite IRA disarmament in 2005, Paisley's rivals say that by leaving the door open to power-sharing he has broken a long-time pledge never to "share power with terrorists".

Sinn Fein faces challenges by several former supporters who say it was wrong to vote recently to support the police, dominated by the Protestant majority.

One Republican who broke with Sinn Fein to stand on an independent ticket was arrested on Thursday and accused of trying to kill a policeman in 1981, local media said.

Police did not confirm the arrest of Gerry McGeough, 46, but said a man had been arrested over a "serious terrorist crime". (Additional reporting by Adrian Croft in Brussels)

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