Ian Paisley's wife and family maintained a bedside vigil as the firebrand Protestant minister, one of Northern Ireland's most prominent political figures, remained in a serious condition in hospital on Tuesday.

The former Northern Ireland First Minister, whose unlikely alliance with his bitter Catholic rivals helped belatedly to cement Northern Ireland's peace process, was admitted to intensive care on Sunday.

Paisley, once a fierce opponent of any concessions to Northern Ireland's Catholic community, went on to lead the province's power-sharing government in the twilight years of his career.

The preacher turned politician has a history of heart trouble and fell ill with heart problems in 2005, writing afterwards that he was walking in death's shadow.

In February last year he had a pacemaker fitted after being taken ill in the House of Lords. A spokesman for the Democratic Unionist Party, which Paisley led for 40 years, said he had not suffered a heart attack.

Former political colleagues visited Paisley in the Belfast hospital and passers-by wished the once vilified 85-year-old well.

I don't see eye to eye with Paisley on many things, but I said a prayer for him when I was at Mass today, said Eamonn Fitzpatrick, a Belfast-based Catholic.

Northern Ireland's first minister Peter Robinson and deputy first minister Martin McGuinness, the former Irish Republican Army (IRA) commander with whom Paisley shared power, also called for prayer in a joint statement.

As the leading light of hardline unionism, which wants to maintain links to the United Kingdom, Paisley was blamed by Irish nationalists for fuelling thirty years of sectarian violence with his famously fierce pledges of no surrender.

He consistently opposed a 1998 agreement that forged peace as a sell-out of Northern Irish Protestants' British heritage. But after his party became the province's largest in 2005, buoyed by disaffection with that deal, Paisley entered talks with Sinn Fein, the political wing of the IRA, in a bid to maintain devolved government in the province.

In 2007 he entered government with McGuinness, with whom he forged an unlikely friendship, earning the pair the nickname the chuckle brothers.

His agreement to share power with McGuinness is seen as the final stage in a decades-long peace process that has brought relative calm to the province despite occasional attacks by dissident Irish nationalists.

The process, with its decade of glacial negotiations that gradually built trust and led to a unique form of compulsory coalition government involving all parties, has been hailed as an example for other conflict-hit areas around the world.

The peace process is seen as a major foreign policy success of former British Prime Minister Tony Blair and U.S. President Bill Clinton, who visited the province several times.

Paisley retired from politics in 2010 and preached his final sermon at his church in January.

(Editing by Padraic Halpin and Andrew Heavens)