Northern Ireland Secretary Theresa Villiers has condemned the "disgraceful acts of violence" in Northern Ireland, in a statement to the Commons on 11 December 2012.
Trouble has flared across Belfast in the past week, with 29 police officers injured and 38 people arrested.
Loyalist protests broke out following a decision by Belfast City Council to fly the union flag at City Hall on designated days, not daily.
On Monday, a petrol bomb was thrown into a car as a policewoman sat inside, which police are treating as attempted murder.
Ms Villiers told the Commons there "can be absolutely no excuse or justification for this kind of thuggish and lawless behaviour".
She said those engaged in violence were "dishonouring and shaming the flag of our country ... and discrediting the cause they claim to support".
The Alliance Party MP for East Belfast, Naomi Long, received a death threat last week.
Speaking in the Commons, Ms Long asked Ms Villiers whether the violence was being treated as a matter of national security.
She also wanted to know whether UK Prime Minister David Cameron would meet the Northern Ireland justice secretary to discuss the security situation.
Ms Villiers responded: "There is a technical definition of national security but whether one applies that in this case or whether one doesn't, it is completely unacceptable for these attacks and threats to take place."
She said she was happy to meet the justice minister at any time and to pass on his request to Mr Cameron for a direct meeting.
DUP MP William McCrea "unreservedly" condemned the violence in Northern Ireland.
But he said it was not right to "demonise" those who wanted to peacefully protest against the decision to remove the flag from Belfast City Council.
"That flag is the very core of the unionist identity and many people of Northern Ireland died in order to keep it aloft in our province," he said.
Ms Villiers said the right to protest peacefully was a fundamental part of democracy but "that cannot, of course, justify" the violence of the past week.
Shadow Northern Ireland secretary Vernon Coaker said the Commons must send a "clear and strong message" that the violence is "wrong, unacceptable and without justification".
Liberal Democrat MP for Eastbourne Stephen Lloyd praised the "dignity" with which Ms Long had dealt with the "very challenging scenarios" of recent days.