The time is now right to support the PSNI, Sinn Fein leader Gerry Adams has told delegates at the Sinn Fein conference (ard fheis) in Dublin.
Gerry Adams prepares his conference speech
More than 2,000 people are attending the meeting, with about 900 delegates set to vote on whether Sinn Fein should support policing in Northern Ireland.
Sinn Fein support for policing and DUP commitment to power-sharing are seen as essential to restoring NI devolution.
"I'm appealing to you to support the motion," Mr Adams told delegates.
"It's bigger than us. It's bigger than Sinn Fein. It's all about the common good," he said.
"Let us go out of here and ready for the next phase of our struggle."
Mr Adams said the party had to become involved in policing to ensure that the kind of collusion outlined by the Police Ombudsman this week could never happen again.
He paid tribute to Raymond McCord, the man whose complaint led to ombudsman Nuala O'Loan's investigation.
The Sinn Fein leader said the party could not afford to leave policing to the unionist parties, the SDLP or the British and Irish governments.
The party's executive decided earlier this month to go ahead with the ard fheis.
It had earlier been put in doubt after the party complained about the lack of a "positive response" from the DUP.
There was a small protest by Republican Sinn Fein supporters at the entrance to the conference venue. "Traitor" was shouted at SF MP Martin McGuinness".
The delegates will have to consider an ard comhairle (party executive) motion backing the police in the event of devolution being restored, and 18 amendments, some of which heavily qualify the Sinn Fein leadership's initiative.
Northern Ireland Secretary Peter Hain said there must be a power-sharing commitment from the DUP and Sinn Fein before the go ahead for a March election.
Without this "there's no point at all in having an election on March the 7th for an assembly (interference) and an incoming executive that won't exist", he said.
Chief Constable Sir Hugh Orde said: "I think it's pretty inevitable that the ard fheis will vote in support of policing. Certainly, over the last five years the reforms we put in place under the Patten reform programme have shown very clearly we are more than fit for purpose."
DUP MP Jeffrey Donaldson said his party's response would depend on how Sinn Fein implemented its policy change.
"The DUP has made clear that whilst we acknowledge the meeting today as very important and will represent Sinn Fein hopefully crossing the rubicon to fully embrace the democratic process.
"From a background of involvement and support of terrorism, nevertheless, what we need to see is the words of today's meeting translated into the actions that are required on the ground to tackle crime and ensure that we have safer communities to live in.
"That means that Sinn Fein have got to encourage people in the community to support the police and the rule of law."
Prime Minister Tony Blair said the move would open the way to real power-sharing for the first time.
Victims' campaigner Wille Frazer protested outside
"And what a fantastic thing that would be. Instead of waking up as we used to years ago to violence and terrorism in Northern Ireland, we have the prospect of peace," he said.
One of those who spoke in favour of the motion was Sean McGlinchey, brother of Dominic and Paul, who is to stand against Sinn Fein on an anti-policing ticket in the forthcoming assembly election.
The vast majority of delegates have echoed the Sinn Fein leadership's view that it is time to get involved in policing.
Those speaking in favour have included Councillor Paul Butler who was jailed for murdering an RUC officer, and Bridget Rose Dugdale, an English heiress jailed for hijacking a helicopter and taking part in an audacious art robbery in the 1970s.
The brother of an IRA man killed by the British army in a gun battle in County Fermanagh in the 1980s voiced his opposition. He said republicans should stick to what he called "guerilla politics".
Mr Blair and Irish Premier Bertie Ahern have identified Sinn Fein support for the Police Service of Northern Ireland (PSNI) as being crucial to persuading the DUP to share power in a devolved government with Sinn Fein by 26 March.
If an election does not occur, Stormont will be dissolved indefinitely.
The transitional assembly at Stormont will dissolve on 30 January in anticipation of an election on 7 March.