Belfast City Council could face legal action over a decision to restrict the flying of the Union flag which has sparked violent protests, the Democratic Unionist Party said.

The change to only hoist the flag on Belfast City Hall on designated days like royal birthdays was endorsed by nationalists and the cross-community Alliance Party at the start of December. Since then, dozens of police officers have been injured by missile-throwing loyalists as ugly scenes erupted in Belfast and other parts of Northern Ireland.

The DUP opposed the change and has lodged an equality complaint with the council.

John Hussey, secretary of the DUP group on Belfast City Council, said the party has submitted a formal complaint to the council that it is in breach of its equality scheme.

He said: "We believe that, both in its processes and in its decision over the removal of the Union flag, Belfast City Council made multiple breaches of its published and legally required equality scheme. This is the first step towards a formal complaint to the Equality Commission and ultimately possible legal action."

Until last month, the flag flew every day of the year from the City Hall. Disputes over flags and symbols have been a regular occurrence in Northern Ireland and the shift to flying the flag on 18 designated days a year has sparked some of the most sustained unrest since the signing of the Good Friday Agreement in 1998.

Despite weeks of demonstrations in parts of east Belfast, some of which have descended into full-scale riots with bricks, golf balls and other objects launched at police lines, councillors have vowed not to reverse their decision on the flag. Sixty-six Police Service of Northern Ireland officers have been injured in violence linked to members of paramilitary organisations.

The Alliance Party, which controls the balance of power on Belfast council, has said the designated days option was in line with the recommendation from Northern Ireland's Equality Commission as an approach which would promote good relations between both sides.

A council spokeswoman confirmed it had received a formal complaint from the DUP about an alleged breach of its equality scheme. She said: "The decision to introduce the policy of flying the Union flag at City Hall was taken democratically by elected members at the monthly meeting of Belfast City Council on December 3.

"The council has taken legal advice throughout this process and the decision is in keeping with the outcome of the equality impact assessment that was undertaken in line with the advice of the Equality Commission. The designated days agreed are in keeping with those notifed by the UK Government's Department of Culture, Media and Sport."