A bomb which exploded at Indonesia's national police headquarters has heightened fears of new attacks in the country, the police chief said.

General Da'i Bachtiar, speaking to legislators just hours after the Monday morning explosion, said the incident had "a huge political impact" in a country still recovering from the Bali blasts last October 12 which killed more than 190 people.

The Bali blast is blamed on regional terror group Jemaah Islamiyah, whose alleged spiritual leader Abu Bakar Bashir is detained at the headquarters in the southern Jakarta suburb of Kebayoran.

No one was hurt by the explosion outside a function hall inside the police compound at around 7:15 am. But legislators asked Bachtiar how such an incident could happen at national police headquarters.

"I would like to apologise for this incident because it has caused public unease, particularly because the bombing raised the fear of new attacks to new heights," Bachtiar told the scheduled meeting of a parliamentary security committee.

The bomb "is a small incident but it has huge political impact," he said. "I'm sure there is some sort of political message," he added, without elaborating.

The blast broke several windows, damaged the entrance and blew a big hole in the wooden ceiling of the function hall lobby. At least one vehicle was damaged.

National police spokesman Edward Aritonang said a policeman found a black bag outside the hall, which can be rented by the public, at about 6.30 am. The "low explosive" bomb exploded while the area was cordoned off. "This is a weakness of our security system," he said.

Asked if the blast might be related to Bashir's detention there, Aritonang said that "all possibilities are being taken into consideration."

Forensic scientists gathered evidence and placed items in plastic bags. Aritonang said police are checking details of a party which rented the hall for a wedding Sunday evening, including the catering and cleaning service.

Bachtiar said the bomb used a timer. "We are trying to unravel this case as quickly as possible with hopes we can find the perpetrator and the motive," he told the legislators.

He told reporters the bomb was made from plastic pipes stuffed with explosive powder and sealed with cement.

Asked if he believes the bombing was a warning related to the Bali investigation, Bachtiar said: "For sure we are not afraid of terrorist threats and we have to face them because that's our job."

Police have arrested 30 people over the Bali bombing – five main suspects, four would-be suicide bombers, and 21 people who assisted the five.

Monday's bombing was the first attack in the capital since a man hurled a grenade near a US embassy residence last September.

The blast missed the house but killed the man who handled the grenade and injured the driver of their vehicle. The driver was later arrested. Three people including the bomber were killed last December in a blast at a McDonald's restaurant in the eastern city of Makassar. Local police said the group which staged that attack is part of the network that carried out the Bali bombing.

Police have said they want Bashir to face charges of treason and of staging a series of bomb attacks on Indonesian churches on Christmas Eve 2000. They said last week that the Muslim cleric is also likely to be declared a suspect in the Bali blast.