They also obstructed the motorcade of Leader of the Opposition and AL chief Sheikh Hasina, who was heading for her party headquarters on Bangabandhu Avenue on hearing the mayhem, in four points, triggering widespread chaos and confusion.

Police said they seized a revolver and 14 cocktails from the AL office in a claim contradicted by journalists on the scene.

Home Minister Altaf Hossain Chowdhury issued a sharp warning to the main opposition in the wake of the violence, saying the government would not allow attack on law enforcers.

When Hasina reached Bangabandhu Avenue at 7:05pm, 80 minutes after taking the troubled journey from her Dhanmondi house, a brownout plunged the area into darkness.

Witnesses said the mayhem broke out at about 4:15pm when a hand-made bomb went off in front of the Muktijoddha Sangsad office and police in force swooped on the activists of Jubo League, youth front of the AL, who were preparing to hold a scheduled rally on nearby Bangabandhu Avenue.

The bomb injured Havildar Nur Mohammad, 35, who formed part of the four-platoon police deployment near the AL central office, prompting his colleagues to go on the rampage.

The law enforcers defied requests for calm from senior AL leaders including Abdur Razzak and Rahmat Ali and surged into the AL headquarters in riot gear breaking open the locks.

They forced their way into every room of the five-storey building and beat up the opposition leaders and activists indiscriminately.

Although Nur could not give the direction of the bomb's flight, his colleagues claimed AL activists hurled it dismissing witness accounts that two young men exploded the bomb, coming out of the Pir Yamini market.

As the leaders and activists of the AL and its front organisations were trying to hide in the washrooms and other apparently safe places in the party headquarters, policemen dragged about 150 of them onto their vans after beating.

Coming to the ground floor from upstairs of the AL headquarters with a black bag, police claimed that it contained the arms and explosives, but journalists on the scene reported they saw police to dump some ink pots into a bucket of water from the bag.

As journalists contradicted the police claim, the law enforcers asked them to leave the scene.

Hasina's motorcade was obstructed at Science Laboratory, Shahbagh, Matshya Bhaban crossings and at Zero Point, before she reached the headquarters to be cordoned off by policemen amid the brownout.

Talking to reporters on the scene lit up by a rechargeable light, she said the drama was staged to divert the nation's attention from the attack on prominent writer Humayun Azad.

She accused the prime minister, home minister and state minister for home of masterminding the scheme to unleash policemen on opposition activists.

The home minister in the Jatiya Sangsad statement said eight to 10 bombs were hurled at the police from the AL headquarters.

He also focussed on the arms recovery and said: "We'll not tolerate any attack on police. Soon we'll arrest the people responsible for the attack."

Colleagues rushed Nur to Dhaka Medical College Hospital where he was undergoing treatment.

Doctors said he was out of danger.

"I don't know where the bomb came from," Nur told The Daily Star.