Alliance MP Naomi Long has been issued with a death threat after her cross-community party backed the move.
On Monday night a policewoman guarding Ms Long's Belfast East constituency office was forced to flee her car after a petrol bomb was thrown into her vehicle in violence which has so far seen 32 police officers hurt and 38 people charged.
Ms Long said the attack on the officer was "a reprehensible attack on democracy".
She added: "The police have stated there is evidence of loyalist paramilitary involvement in some of the protests and violence in Northern Ireland this week."
Speaking at Prime Minister's questions in the Commons, Mr Cameron said: "In no way are these people being loyal or standing up for Britishness.
"Violence is absolutely unjustified in those and in other circumstances."
He told Ms Long she had MPs' "complete solidarity" and praised her political work amid intimidation and death threats.