Rallies were held in Belfast, Lisburn, Newry, Downpatrick and Derry showing solidarity against two deadly attacks claimed by republican dissidents.
Tonight hundreds of people attended a peace vigil in Craigavon in Co Armagh near the site where Constable Stephen Carroll was shot dead on Monday.
At lunchtime in Belfast, the streets around City Hall were brought to a standstill as thousands of people gathered for a rally organised by the Irish Congress of Trade Unions.
Protestors held a couple of minutes' silence and a lone piper played 'Amazing Grace' and 'Abide With Me'.
'The callous attacks of the last few days were an assault on every citizen who supports peace,' Peter Bunting, the ICTU's assistant general secretary, told the crowd.
There had been minor disturbances in Craigavon last night, a short distance from where Constable Stephen Carroll was killed.
Youths pushed a number of wheelie bins onto two roundabouts in the Ardowan area a few hundred metres from the scene of the shooting.
Police warned motorists to keep away from the area and monitored the situation in the hope that it would not escalate.
In a separate development, Sinn Féin's Tom Hartley, Lord Mayor of Belfast, held talks with members of the Ulster Political Research Group, which has links to loyalist paramilitaries in the UDA.
Meanwhile, a 37-year-old man and a 17-year-old youth are still being questioned in connection with the killing of the 48-year-old PSNI officer, who was shot dead by the Continuity IRA.
The arrests were made in Craigavon yesterday as police officers searched a number of homes in the Drumbeg estate, which overlooks the scene of the shooting.
Elsewhere this afternoon about two dozen MPs from across the party divide stood in silence outside the Houses of Parliament in London in solidarity with those who have been taking part in the silent vigils against the killings in Northern Ireland.
The Irish Government was represented by the Charge d'Affaires at the London embassy Mr Kevin Conmy.
Earlier, Pope Benedict XVI condemned the recent violence.
'I condemn in the strongest terms these abominable acts of terrorism which, apart from desecrating human life, seriously endanger the ongoing peace process in Northern Ireland,' said the Pontiff at his weekly general audience.
'I ask the Lord that no one will again give in to the horrendous temptation of violence,' he said.
