Media playback is unsupported on your device Media caption The BBC's Mark Simpson said the timing of the protests was significant

Four police officers have been injured and Alliance Party properties targeted during trouble involving loyalists in Northern Ireland.

Some 1,500 people gathered in the town to protest against Belfast City Council's decision to stop flying the union flag every day.

Earlier, an Alliance Party office in Carrickfergus was destroyed.

The home of a couple who are both Alliance Party councillors in Bangor, County Down, was also attacked.

Paint was thrown at Michael and Christine Bower's home, smashing a front window close to where their daughter normally plays.

No-one was injured although the couple said they were shocked by what had happened.

The violence came during an important week for Northern Ireland as US Secretary of State Hillary Clinton is due to visit on Friday.

About 1,500 people had gathered at the Irish Gate roundabout in Carrickfergus earlier on Wednesday, police said, to protest against the council's decision.

The crowd dispersed but some people stayed behind, trouble broke out and missiles, bottles and masonry were thrown at police.

Image caption An Alliance Party office was ransacked during the trouble

Rioting continued in nearby West Street, where Alliance MLA Stewart Dickson has a constituency office.

It was ransacked and the rioters tried to set it on fire. The building was smoke damaged.

Mr Dickson said he had received hundreds of calls and texts from people across the country who were "sickened by the violence".

He appealed to Mrs Clinton to come and visit his office during her visit to Northern Ireland.

"I want Hillary Clinton to see at first hand what a full frontal attack on democracy looks like," he said.

Police fired a number of baton rounds at the crowd. They said a number of officers were injured in the riot but their injuries were not thought to be life threatening.

Analysis Alliance are saying the two main unionist parties have to bear responsibilities. Sammy Wilson, DUP, said the people involved in the violence did a disservice to the flag they were supposedly trying to protect. Mike Nesbitt, UUP, talked about unionism needing brains not brawn. But it didn't take long before we had cantankerous exchanges between them. Sammy Wilson heard Alliance leader David Ford, effectively accusing him of hypocrisy. He came back with this assertion that Alliance should have known that opening this Pandora's box would lead to something like this and if they didn't they were out of touch.

PSNI Assistant chief Constable Will Kerr said the protest was organised through social networking sites.

"What we saw in Carrickfergus was the result of bringing large numbers of people out onto the street where we can't control those large numbers of people," he said.

"We saw criminal, thuggish, intimidating behaviour here. We need to put a stop to this very quickly."

By 22:20 GMT on Wednesday, police said the trouble had come to an end, although they were maintaining a presence in Carrickfergus.

Two men and two women have been arrested in connection with the violence.

Assembly call

Michael Bower, a North Down councillor, said he was sitting in his living room at about 23:00 GMT on Wednesday when the attack happened. The device shattered a pane in the double-glazed window of the front living room.

"My daughter who is 17-months-old had just gone to bed when I heard an explosion against the window," he said.

"I guessed it was some sort of attack on our family home. I checked that my wife and daughter were okay. The paint bomb hit the corner of the room where my daughter usually plays.

"There is damage to the window, but it could so easily have been so much worse. If events had been different, a young life could have been lost. I would plead with people to calm down and to stop attacking representatives doing their best to support their communities."

North Down Alliance MLA Stephen Farry said there was an attempted arson attack on his constituency office in Bangor.

"Petrol or some other accelerant was poured over my shutters by a number of people," he said.

"Fortunately, they were interrupted by a passing police patrol and fled the scene."

Meanwhile, Alliance leader David Ford has requested the Northern Ireland Assembly be recalled on Thursday to discuss the attacks.

He said what had happened was "a direct assault on democracy of the worst possible kind".

"I believe there is now a heavy responsibility on Peter Robinson and Mike Nesbitt to provide an alternative kind of leadership which would divert people away from the kind of trouble," he said.

'Brains, not brawn'

DUP MP Sammy Wilson has condemned the attack in Carrickfergus.

"People are entitled to peacefully protest but there is absolutely no justification for the carnage which has been caused in Carrickfergus this evening," he said.

"Those involved do a disservice to the flag they claim to defend. Today, in the House of Commons, several MPs backed the union flag being flown from city hall, however this criminal behaviour undermines and distracts from the argument.

"I urge those involved to step back from this criminal activity and channel their energies through democratic politics."

Ulster Unionist leader Mike Nesbitt MLA said violence was legally and morally wrong.

He said: "What unionism needs today is brains, not brawn. We need an analysis, a strategy and an agreed outcome. I am happy to talk to anyone, if they commit to repent and desist from violence."

Jim Allister of TUV said the violence was "utterly wrong" and was not the way forward.

A loyalist flag protest also took place in Lurgan.