Politicians and communities from across the religious divide united yesterday in condemnation of dissident republican paramilitaries who shot dead two British soldiers outside their barracks near Antrim hours before their regiment was due to leave the province for a tour of duty in Afghanistan.

In a historic moment for Northern Ireland, Sinn Féin's Gerry Adams and Martin McGuinness last night backed the police manhunt and denounced the attackers. McGuinness, the deputy first minister, said: "I was a member of the IRA, but that war is over now. The people responsible for last night's incident are clearly signalling that they want to resume or restart that war. Well, I deny their right to do that."

In an emotional interview, he said: "I will stand for all democrats against their attempts to plunge us back into conflict; to see soldiers on the streets; to see more checkpoints; to see houses being raided and to see people being dragged back to interrogation centres. Those days are over. They can never come back again."

In an unprecedented statement, Adams said: "Sinn Féin has a responsibility to be consistent. The logic of this is that we support the police in the apprehension of those involved in last night's attack."

As political leaders denounced the attack and pledged to keep the peace process on track, it emerged that the soldiers from 38 Engineer Regiment had been dressed in their desert fatigues, ready to fly to Afghanistan. Two were killed and four other people injured after machine gun fire opened up on the barracks entrance as two cars brought a delivery of pizzas to the soldiers, who had ordered the food after being told their flight was delayed. The two soldiers, from London and Birmingham, were shot again as their attackers stood over them. The two pizza delivery men were among those injured, one critically.

Last night a reporter working for the Sunday Tribune in Belfast received a call with a recognised code word said to be from the Real IRA, claiming responsibility for the attack. The same organisation claimed responsibility for the Omagh bombing which killed 29 people in 1998.

In its statement, the Real IRA said it did not apologise for shooting the pizza delivery workers, because they had been "collaborating" with the British army.

The main road past Massereene barracks was reopened last night and more floral tributes were gathering beneath the two-storey security tower at the entrance to the military base. The attack on Sunday night was the first time British troops had been killed in Northern Ireland by terrorists since 1997. But Northern Ireland has been on alert over the growing threat from an estimated 200-300 dissidents thought to be active in the province. Sir Hugh Orde, head of the police service in Northern Ireland, confirmed last week that the level of threat from the Real IRA and other groups was at critical level, the highest since he took charge seven years ago.

Sunday morning prayers at churches across Antrim town were cut short as congregations came together in an ecumenical show of solidarity. Hundreds walked silently to the edge of the police cordon and stood together amid floral tributes at the Massereene barracks.

The attack was condemned by the prime minister, Gordon Brown, as ministers made a concerted effort to dismiss it as the product of a small, self-obsessed group of thugs with no political support in Northern Ireland.