Sinn Fein's offices at the Northern Ireland Assembly have been raided as part of a major police investigation into intelligence gathering by republicans.

It is believed a large quantity of documents was seized and three people detained in the searches that took place in north and west Belfast.

Among those being held is a former junior employee of the Northern Ireland Office and Sinn Fein's head of administration, Denis Donaldson.

Sinn Fein President Gerry Adams said he had complained to the British Government about the searches.

Police leave Stormont buildings after the raid

He accused the police service of being "anti-peace process, anti-Sinn Fein and anti-democratic".

The raids began at about 0500 BST on Friday.

The Police Service of Northern Ireland said the search at Stormont was "confined to one desk and immediate surrounding area".

They also said a number of computer discs were "seized for further examination and an officer of the assembly was present throughout search".

A number of premises in the north and west of the city were also searched and a number of items were seized.

Speaking at Stormont, Sinn Fein's Gerry Kelly condemned the raids on his party's offices.

He said: "It's about blackening Sinn Fein to let David Trimble off the hook - it is politically unbelievable."

The Ulster Unionist leader urged the British Government to act following the latest development.

David Trimble said it would have "grave implications" for the future of the power-sharing government at Stormont.

He said he suspected IRA intelligence was "directed at the upper echelons of the government, having penetrated the Northern Ireland Office".

He added: "This is on a par with the activities we believe republicans have been engaged in for some time.

Gerry Adams: Complaint to government over raids

"We have long suspected that this sort of activity is going on."

He said that was part of the reason why he gave Sinn Fein the 18 January deadline for proving the republican movement was committed to peace, or his ministers would quit the executive.

Security sources said the searches and seizures were not connected to the investigation into the theft of intelligence files at Castlereagh police complex on 17 March.

A Sinn Fein spokesman said that homes of "community activists" in west Belfast had also been raided on Friday morning.

The spokesman said documents related to policing had been seized.

On Friday afternoon, about 60 people held a protest at the headquarters of the Policing Board in Belfast against the raid at Sinn Fein's Stormont office.

Stormont, in east Belfast, houses the Northern Ireland Assembly.

Sinn Fein has about 18 offices there.