The attack occurred outside the office of Naomi Long, a member of the British Parliament, the Belfast Telegraph reported. Long is a member of the Alliance Party, which crafted a compromise limiting the display of the flag at Belfast City Hall to 17 days a year.
Loyalists in Belfast and elsewhere in Northern Ireland turned out to protest Monday night, a week after the City Council voted on the flag. Firebombs and fireworks were used against police in south Belfast as well.
George Hamilton, an assistant chief constable with the Police Service of Northern Ireland, described the attack outside Long's east Belfast office as attempted murder.
"This was a planned attempt to kill a police officer which also put the lives of the public in danger and it is fortunate there were no injuries," Hamilton said.
The British flag has been flown over City Hall every day for more than a century. Nationalists wanted to ban it completely, but Alliance proposed flying the flag on occasions such as the official celebration of Queen Elizabeth's birthday.