The force of the blast, which occurred at 8.30 am (0130 GMT), blew concrete and nails across a wide area, causing minor damage.
Police said an explosive device was placed near an air-conditioning unit outside a function venue close to the main parliamentary auditorium.
Officials said details were sketchy at the moment, but Jakarta police chief Makbul Padmanegara told reporters: "Yes it was a bomb. It was quite strong. I can't explain at the moment what type it was."
The explosion was the latest blow to a regime grappling with Muslim opposition groups and ethnic rebellions. It came days after police claimed to have foiled plans by Islamic groups to attack churches and shops in Jakarta by arresting nine suspected members of Southeast Asian Muslim network, Jemaah al Islamiah (JI).
Bali bombing trial
And the blast coincided with the trial of a key suspect in the Bali bombings which killed scores of Western holidaymakers last year.
Amrozi, presenting a defence plea at his trial on Monday, said the blasts which ripped through two crowded nightspots and killed 202 people also prevented the economy of the resort island from falling into the hands of foreigners.
Bali bombing suspect Amrozi
Amrozi, a village mechanic, is accused of buying one tonne of chemicals to help make the bomb which blew the Sari Club to bits, along with the van which carried the bomb.
Reading out his plea, he said: "I am also not among those who are against tourists, and tourist arrivals should even be promoted but on condition that they follow disciplines."
Threat of foreigners
"They, as guests, should follow our rules and not us follow their rules just because of money."
He added that the bombing would turn people back to Islam.
"With this incident, God willing, many people realise that they had forgotten God and neglected their worship and avoided places of worship so that mosques became empty, churches became deserted, monasteries and temples also became empty without occupants or visitors," he said.
During his trial Amrozi has described foreign tourists as a threat to Indonesia's future and said violence was the only language they understand. However, he said he did feel remorse for the 38 Indonesians who died in the blast.
He also repeated earlier claims that he invented many of his admissions to police, including reported meetings to plan the bombings, because he felt "forced to" during tiring and lengthy interrogation sessions.
His defence lawyers, reading their own plea, said Amrozi may have purchased the chemicals for the bombs, but was certainly not a planner of the attack.
"The defendant, has never been proven, legally and convincingly, to have been the planner or the executor of the Bali bombings," defence lawyer Yusuf Adnan said.
Police say Jemaah al Islamiyah (JI) staged the Bali blasts to avenge perceived injustices to Muslims worldwide. However, security analysts said it was too soon to connect the parliament attack with the group.