Ian Paisley Jr, a member of the Policing Board, said afterwards: "This could be a defining moment in the history of Northern Ireland."
The attack happened just 36 hours after Northern Ireland Chief Constable Sir Hugh Orde confirmed that undercover soldiers had been called in to carry out surveillance operations on dissidents amid warnings that the threat against his officers and military personnel was at its highest for almost a decade.
Witnesses reported hearing two long busts of gunfire as a car drove by the barracks.
At least six ambulances and three paramedic vehicles rushed to the scene as emergency sirens blared from inside the complex.
The injured were taken to Antrim Area Hospital, about a mile away.
The area around the barracks was sealed off and a massive security operation was under way.
Nobody has yet claimed responsibility for the attack.
Last year dissident Republicans tried to kill PSNI officers in separate incidents in Derry City and Dungannon Co Tyrone.
Last month security forces also defused a 300lb bomb in Castlewellan Co Down which may have been intended for an attack on a nearby barracks.
Mr Paisley, a Democratic Unionist member of the Northern Ireland Assembly, said: "For the last 10 years, people believed things like this happened in foreign countries, places like Basra. Unfortunately it has returned to our doorstep.
"There are people who have been intent on murdering police officers or soldiers, or someone else, to strike home and galvanise support for some mad cause. This is where we are tonight.
"Some people also tried to exaggerate that message, and if this shooting is attributed to dissident republicans, then it was no exaggeration.
"The police have managed to keep the lid on this and they have had some successes against these people which they've kept quiet. They have disrupted and harried them, but its the people of Northern Ireland who will suffer."
Security chiefs had warned for months that dissident republicans were determined to inflict fatalities. Five police officers, two of them off-duty, were wounded in separate gun and bomb attacks in Londonderry, Dungannon, Co Tyrone, near Castlederg, also Tyrone, and not far from Roslea, Co Fermanagh, where the threat has been at its highest.
There have also been a series of failed bomb attacks, one just a few weeks ago near Castlewellan, Co Down.
The last soldier to be murdered in Northern Ireland was Lance Bombardier Stephen Restorick, 23, who was shot by an IRA sniper at a checkpoint in Bessbrook, County Armagh in February 1997.
The 38 Engineer Regiment has been based at Massereene Barracks since August last year but is due to be relocated to RAF Aldergrove in Co Antrim by the end of 2010.
The relocations were announced in April last year by Armed Forces Minister Bob Ainsworth who said Massereene Barracks would then be closed and disposed of.
Kylie McLaughlin, who lives near the scene, told the BBC: "It was constant fire like a machine gun.
"It was very scary, we were not sure what was happening. We just can't believe it has happened here."
The SDLP's Thomas Burns MLA said: "We condemn this attack.
"This has taken us back to the bad, bad old days which we have left long since behind.
"It will cause the power sharing executive to wobble a bit - we are all very, very concerned about that."
He added: "We are interested about moving Northern Ireland forward.
"We want to see a prosperous Northern Ireland, we don't want to see people getting shot in the street."
Northern Ireland's First Minister and Democratic Unionist Party leader Peter Robinson said he was postponing a trip to the United States as a result of the shootings.
He said they were a "terrible reminder of the events of the past".
Mr Robinson added: "These murders were a futile act by those who command no public support and have no prospect of success in their campaign. It will not succeed."
Some reports suggested the attackers arrived at the barracks in a taxi vehicle to evade suspicion.
The SDLP condemned the attack which it branded as brutal and despicable.
South Antrim MLA Thomas Burns said: "Like many, we had hoped and prayed that nights like this were firmly in the past.
"It is important we unite as a community and send out a message to those responsible that they have absolutely no support.
"Tonight the two families are in mourning as a result of this despicable attack."
He appealed to anyone with information to contact the police.
It emerged last week that intelligence service MI5 was still allocating 15% of its resources in 2007-2008 to anti-terrorism activities in Northern Ireland.
The Intelligence & Security Committee Report also said that dissident republicans continued to pose a threat in Northern Ireland and the rest of the UK.
He said: "My thoughts are with the families of those killed and injured in this murderous attack. The Prime Minister is being kept closely informed.
"The contrast between those who serve the community and those who would destroy it could not be clearer.
"The people who did this will be pursued and they can be assured that they will never be able stop political progress in Northern Ireland."
Downing Street added in a statement::"This is a terrible incident that we utterly condemn and the prime minister's thoughts, first and foremost, are with the families of those killed and with those seriously injured in this attack.
"In recent days action has been taken to increase security in Northern Ireland. This is because of the increased threat from those who, even at this late stage, wish to ignore the wishes of the overwhelming majority of the people of Northern Ireland and attempt to derail the peace process.
"The full facts of this incident are being investigated. We will do everything we can to ensure those responsible are brought to justice."