Between six and eight devices have been defused at Stormont after loyalist Michael Stone stormed the building, Sir Hugh Orde has said.
The Northern Ireland police chief said "their potential for death, destruction and injury is being assessed" but added they were "fairly amateurish".
The convicted killer attempted to enter Stormont during a key debate to pave the way for restoration of devolution.
Assembly security staff disarmed him as he entered the building.
He was carrying a gun, knife and a suspect device.
Michael Stone is being questioned about what Sir Hugh branded "a sad publicity act by a very sad individual".
In 1988, Stone murdered three men at the funerals of three IRA members.
The Stormont meeting was being held to hear if the DUP and Sinn Fein would indicate ministerial candidates.
The alert happened about 20 minutes into the proceedings.
It is understood Stone threw a bag into the hallway, was detained by civilian security guards and Northern Ireland politicians were quickly ushered out of the building.
Northern Ireland Secretary Peter Hain has ordered an urgent report from Chief Constable Sir Hugh Orde into the breach of security.
Prime Minister Tony Blair said that despite the breach, the St Andrews Agreement remained the only way forward.
Speaking from Downing Street, Mr Blair said: "No move forward in Northern Ireland is easy, we've learned that over 10 years.
"It's not because the people, or indeed, the leaders in Northern Ireland want it to be so, but because each step towards a different and better future is taken alongside the memory of a wretched and divisive past."
BBC political correspondent Gareth Gordon said it looked as if the building would remain evacuated for the rest of the day.
Friday had been billed by the two governments as a "critical day", with politicians gathered to hear if the DUP and Sinn Fein would indicate their candidates for the first and deputy first minister jobs.
During the debate, Sinn Fein said Martin McGuinness was its choice for deputy first minister.
In his speech, Mr Paisley said the circumstances had not been reached where there could be a nomination or designation by his party.
"There can only be an agreement involving Sinn Fein when there has been delivery by the republican movement, tested and proved over a credible period in terms of support for the PSNI (the police), the courts, the rule of law, a complete end to paramilitary and criminal activity and the removal of terrorist structures," he said.
"Clearly, as Sinn Fein is not yet ready to take the decisive step forward on policing, the DUP is not required to commit to any aspect of power-sharing in advance of such certainty."
WHO IS MICHAEL STONE?
Stone murdered three men at the 1988 funerals of three IRA members killed by the SAS
He was released early in June 2000, under the terms of the Good Friday Agreement
UUP leader Sir Reg Empey challenged the Speaker, Eileen Bell, as to whether DUP leader Ian Paisley had actually indicated his party would nominate its choice for first minister.
"It requires to be clarified as to whether or not we have witnessed a marriage or an engagement today," he said.
However, Mrs Bell said that it was now a matter for Northern Ireland Secretary Peter Hain to decide.
Ian Paisley and Gerry Adams both addressed the chamber
Mr Hain said the assembly would meet on Monday and the incident would not be allowed to derail the political process "or the opportunity of getting democratic self-government up and running".
"I thing its very important that the party leaders and the parties now stand together and defend democracy against this kind of violent attack and I hope the assembly can convene quickly on Monday," he said.
"I have just seen the leader of the Democratic Unioist Party, Ian Paisley's statement in which he confirmed what he said at St Andrews, it is his intention to accept the nomination for First Minister, after the people have spoken, after the election on 7 March and provided that all the conditions agreed at St Andrews have been met."
Sinn Fein leader Gerry Adams said that for a "split second" an old agenda had resurfaced, and commended the bravery of assembly staff who had tackled the feared paramilitary.
"Let's not be put off by what occurred here today, but let's take it as an incentive to make sure that the people get what they deserve," he said.
Mr Adams said the assembly should be reconvened as quickly as possible and that "only when the session was finished could a judgement be made".
SDLP leader Mark Durkan said: "There is as much hollow farce as there is historic significance in what we have witnessed this morning".
If all goes to the British and Irish government's plan, assembly elections will be held in March, with devolution restored later that month.
The DUP and Sinn Fein get to nominate first and deputy first ministers as they are the largest unionist and nationalist parties in the assembly.
For months the British and Irish governments billed 24 November as a make-or-break date.
But since last month's St Andrews Agreement, the deadline has been watered down, with no talk of the politicians' wages and allowances being cut.
Friday's meeting of the assembly was the first since legislation was passed to redesignate it as a transitional body which will be dissolved in January, to pave the way for elections in March.
Ahead of the meeting, Mr Hain warned that he was prepared to pull the plug on Stormont unless it seemed that progress could be made.