* Flag protests reignite after Christmas respite
* More police injured, several more arrests made
BELFAST, Jan 4 (Reuters) - Northern Irish police came under attack by pro-British loyalists on Friday as the province's first minister branded rioters "a disgrace" and said they were playing into the hands of rival militant nationalists.
Rioting began a month ago after a vote by mostly nationalist pro-Irish councillors to end the century-old tradition of flying the British flag from Belfast City Hall every day unleashed the most sustained period of violence in the city for years.
On Friday, police said officers came under attack in the east of the city by masked mobs hurling petrol bombs, rocks and fireworks.
A number of officers were injured, several arrests were made and police deployed water cannon to control a crowd t hat a t one point swelled to 400 protesters.
First Minister Peter Robinson, leader of the pre-eminent Protestant group, the Democratic Unionist Party, called the decision to take down the flag "ill-considered and provocative" but said the attacks must end.
"The violence visited on (police) is a disgrace, criminally wrong and cannot be justified," said Robinson, whose party shares power with deputy first minister and ex-Irish Republican Army commander Martin McGuinness' Sinn Fein Party
"Those responsible are doing a grave disservice to the cause they claim to espouse and are playing into the hands of those dissident groups who would seek to exploit every opportunity to further their terror aims."
More than 40 police officers were injured in the initial wave of fighting, which stopped over Christmas, only to resume on Thursday when a further 10 police officers were hurt as the community divisions were exposed once more.
At least 3,600 people were killed during Northern Ireland's darkest period as Catholic nationalists seeking union with Ireland fought British security forces and mainly Protestant loyalists determined to remain part of the United Kingdom.
Anti-British Catholic dissident groups, responsible for the killing of three police officers and two soldiers since 2009, have so far not reacted violently to the flag protests, limiting the threat to Northern Ireland's 15-year-old peace.
Another demonstration calling for reinstating the Union Flag will be held outside City Hall on Saturday while some loyalists have pledged to hold a protest in Dublin the following Saturday.