Police officers collect evidences at the site of a suicide car bombing in Islamabad on Friday. Reuters

A policeman was martyred, two terrorists were killed and at least six people — including four police officers and two civilians — were injured in a suicide blast in Islamabad’s I-10/4 sector on Friday, police officials said.

Shortly afterwards, the Islamabad police declared a "red alert” in the city.

Deputy Inspector General of Police Sohail Zafar Chattha said the police spotted a "suspicious vehicle” with a man and a woman aboard at 10:15am in the area.

"When the police stopped the vehicle, the couple came out of the car. The long-haired man, while being checked by the officers, went inside the vehicle on some pretext and then detonated himself,” he told reporters.

Chattha said a police officer of the Eagle Squad was martyred in the blast while four others officers were injured. He lauded the police for "saving Islamabad from a major foul play.”

The joint executive director of the Pakistan Institute of Medical Sciences (Pims), Dr Iqbal Durrani, said two bodies and five injured policemen had been brought to the hospital.

Interior Minister Rana Sanaullah confirmed that "two terrorists” were killed in the blast.

"The vehicle used in the blast was registered in Chakwal. The car entered Islamabad from Rawalpindi and was packed with explosives and headed for a high-value target in the capital,” he said. Sanaullah added that had the car reached its target, it would have caused heavy losses.

In a tweet, the Islamabad police identified the slain officer as Head Constable Adeel Hussain.

It said that terrorists had been targeting police for some time to demoralise law enforcement personnel. In another tweet, the police said that diversions had been placed for both sides of traffic at Service Road East of Sector I-10/4.

It advised citizens to use Service Road West of Sector I-10/4 as an alternate and stay away from the blast site.

Prime Minister Shahbaz Sharif strongly condemned the blast and sought a report from authorities.

"Due to the timely action of the law enforcement agencies, the nefarious plan of terrorists to shed the blood of innocent people has been foiled,” the premier said in a statement.

The outlawed Tehrik-e-Taliban Pakistan (TTP) claimed responsibility for the attack in a statement released on Friday.

Mohammad Khalid Khurasani, the spokesman for the TTP, said in a statement one of the group's militants carried out the suicide attack to avenge the killing of a senior leader.

Abdul Wali, widely known as Omar Khalid Khurasani, was killed in a roadside bombing in August in Afghanistan’s Paktika province.

His death was a heavy blow to the militant group, who blamed Pakistani intelligence agents for the killing without offering any evidence or elaborating.