Thousands of people gathered today to take part in a rally in Omagh to mark the death of constable Ronan Kerr.
The rally, which was described by organisers as a march for peace, came as detectives continued to question three men over the murder of the 25-year-old Catholic officer, who died in a booby trap bomb attack outside his home in the Co Tyrone town last week.
Deputy First Minister Martin McGuinness today called for communities to "hold steady" in the face of the threat. His comments came after his party president Gerry Adams warned people against offering any shelter to breakaway groups.
Police were granted an extra five days to question a 33 year old man who was arrested on Friday in the Omagh area.
Police were yesterday also given five more days to question a 26-year-old man arrested in Scotland on Wednesday and re-arrested on Thursday, plus a 40-year-old man arrested near Omagh on Thursday.
Meanwhile, police in Northern Ireland confirmed that a van at the centre of a major security alert close to the border yesterday contained a 'very substantial' bomb.
A number of controlled explosions were carried out on the vehicle found at a section of the main road between Belfast and Dublin, near Newry, after which officers confirmed a viable bomb had been found.
It was contained in a wheelie bin packed with 500lbs of homemade explosives and could have been destined for an attack on a town centre, according to the PSNI.
Detectives believe the vehicle, found near Newry at an underpass beneath the main A1 route between Belfast and Dublin, was abandoned because of the presence of a police checkpoint.
There was no immediate claim of responsibility for the bomb, but police said a phone call to warn about the bomb used a code word previously used by dissident Republicans.
There are fears the van bomb abandoned near Newry was intended for a town centre and could have caused loss of life on the scale of the 1998 Omagh massacre.
Mr McGuinness said of the groups: "Over the course of recent years it appears that they do have a limited capability, but you can't rule out anything.
"I think the greatest danger from these groups is to the local community.
"If you look at the people that have been killed by these groups they are mostly from the nationalist/republican community."
In a direct appeal to the small support base for the dissidents, Mr Adams warned against sheltering what he described as anti-peace factions, and he argued that the political landscape had changed since the days of the Troubles.
"To those who might shelter or provide resources and facilities to the perpetrators of these actions you need to ask yourselves what purpose is being served?" he said.
"Don't be fooled into thinking that you are helping the IRA. The war is over."
Additional reporting: PA