The incident happened on the Upper Newtownards Road shortly after 7.30pm on Monday, during the latest loyalist protests over the decision to restrict the flying of the Union Flag outside Belfast City Hall.
Two officers in the unmarked patrol car were guarding the east Belfast office of Alliance MP Naomi Long, who received a death threat last week after the passing of the party's motion by Belfast City Councillors.
The windows of the vehicle were smashed by men armed with iron bars and a petrol bomb was thrown into the car while one officer was still inside. Paint was also thrown at the car.
Police say the officer "was lucky to escape without injury".
Assistant Chief Constable George Hamilton has condemned the attack.
He said: "It is completely unacceptable that officers who were carrying out their professional duty on behalf of the community should be subjected to a potentially murderous attack.
"I am urgently appealing to those involved in ongoing protests to listen to their political leaders and step back from protest activity before someone is seriously injured or killed."
First Minister Peter Robinson has condemned the attack as "a despicable act of terror".
"The masked men responsible do not act in the name of our Union Flag. They are bringing shame on it. My prayers are with the police officers at the centre of this attack," the DUP leader said.
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 as a result of this attack.
ACC George Hamilton
Ms Long said: "My thoughts are with the police officers who were there to serve the community and have been viciously attacked while carrying out their duties.
"There can be no justification for attacks on the police, elected representatives or anyone else.
"If Northern Ireland is to move forward then we need a strict adherence to the rule of law and respect for the democratic process. We need this urgently before lives are lost."
Alliance leader David Ford said there was no justification for any attack on police officers.
"The PSNI are there to serve everybody in the community," he said.
"While I respect the right to peaceful protest, it is clear that there is a need for these protests to end due to the violence and the increased tensions on the streets."
Ulster Unionist leader Mike Nesbitt said there is "no place in society for actions as those witnessed in east Belfast".
"Police officers have been faced with extremely difficult conditions over the past number of days and far from being attacked, they deserve our support," he said.
Disorder also broke out in the Mountpottinger Road and Castlereagh Street area of east Belfast.
Earlier water cannon was deployed in the Village area of south Belfast after officers came under attack.
Missiles, including masonry, fireworks and bottles, were thrown at officers in the Tavanagh Street and Broadway areas.
Around 200 people gathered in the area with flags and placards at Broadway roundabout for a protest which has now ended.
According to UTV reporter Peter Cardwell, who was at the scene, riot police got out of police vans and ran towards protesters who retreated.
Tonight we could have been looking at the deaths of police officers. This was an attempt to kill police officers. This violence must end now.
David Ford, Alliance Party leader
Rush hour traffic throughout the city was hit by major delays on Monday evening, as loyalists blocked a number of main routes.
The Upper Newtownards Road was blocked for a time by hundreds of protesters and police urged people to be aware of the potential impact on the nearby Ulster Hospital.
"Protesters are reminded that this is bringing serious disruption to the lives of the local community and that a peaceful protest can only be facilitated for a limited time," the PSNI spokesman said.
"The priority must be ensuring free access to the Ulster Hospital and preservation of life.
"We would ask everyone to heed police instructions and keep the road clear to ensure access to the Ulster Hospital is not blocked and that traffic is not delayed."
Reacting, Mike Nesbitt said: "There is nothing British about attacking police officers or blocking the entrance to an acute hospital to prevent ambulances getting in and out."
In Lisburn, the Queens Road was blocked by protesters and diversions put in place.
In north Belfast, the Crumlin Road was closed for a time between its junction with Cambria Street and Hesketh Road - also due to a protest in the area.
Police also monitored protests throughout the evening at Finaghy crossroads, Shaws Bridge, Seymour Hill in Dunmurry, and on the Ormeau Road.
Traffic came to a standstill for a time when protesters gathered at Ormeau Bridge, before being moved down Annadale Embankment by police.
Meanwhile, in Ballyclare around 140 people protested at the town's square, while a protest was also held in Limavady. And in Ballycastle, protesters gathered outside Moyle District Council offices.
Motorists in all affected areas were advised to seek alternative routes.
There are children in childcare who are will be distressed by their parent's failure to collect them. Do the protesters want to be known as people who cause distress to young children?
Conall McDevitt, SDLP
South Belfast SDLP MLA Conall McDevitt has condemned the violence and subsequent disruption.
"Whatever political points people want to make by blocking roads and attacking the PSNI will be lost in the anger they have ignited among working people trying to get home to their families," he said.
"Blocking roads serves no practical purpose and undermines any cause people feel they may be furthering and is deeply damaging the reputation of the City.
"Political unionism needs to show leadership to bring an immediate end to these protests."
Meanwhile, the leaders of the DUP and UUP met on Monday evening to discuss the flag issue.
Peter Robinson and Mike Nesbitt held talks for over an hour and, according to a joint statement, "agreed to work on a joint basis with a view to urgently bringing forward political proposals to address widespread concerns across the community".
They say further discussions will be held on Tuesday in order to "finalise a strategy".