A suicide bomber has struck a police station in the Pakistani capital, killing a police officer.

At least two policemen were in a critical condition following Monday's attack on a station that houses police intelligence and bomb disposal units in Islamabad.

Witnesses said body parts were strewn across the ground and that the blast could be heard across much of the city centre.

The attack came amid scaled down ceremonies to mark the country's Republic Day.

Police said the attacker tried to enter the Special Branch office and, when guards tried to stop him, he blew himself up.

Police moved quickly to cordon off the area, which is near the central Sitara market.

Rehman Malik, Pakistan's interior minister, inspected the site of the blast, saying he has been warning of possible attacks for some time.

He told Al Jazeera that up to seven suicide bombers could be in Islamabad, ready to carry out operations.

Al Jazeera's Zeina Khodr, reporting from the scene, said it was still too early to say who was responsible.

She said Pakistanis were expressing their dismay at the inability of security forces to prevent such attacks.

"Islamabad has not been spared in the recent wave of violence across Pakistan, and there have been special security measures all across Islamabad," Khodr reported.

Pakistan was marking Republic Day on Monday, although celebrations and the traditional military parades had been called off due to security concerns.