DHAKA, June 17 (Reuters) - Bangl-adesh police said on Sunday at least 22 people have died of injuries after a powerful bomb exploded in an office of the ruling Awami League political party.

The violence came as Prime Minister Sheikh Hasina prepares to step down from office after her five-year term ends on July 13, with elections under a caretaker authority due by October.

Hasina’s Awami League accuses opposition parties of using armed gangsters to disrupt law and order and kill people in a bid to destabilise the country in the run up to the polls.

Police said 15 people died on the spot when the bomb went off on Saturday night in the town of Narayanganj, 15 km (9 miles) east of the capital Dhaka, and nearly 40 were critically injured.

Narayanganj police inspector Noor Hossain told Reuters on Sunday that seven of the injured died in hospital.

"We saw bodies lying in a pool of blood in the debris of the devastated Awami League office," a police officer told reporters. "Many of the injured had their hands and legs blown off...it was a terrible scene."

"The bodies have been removed but some torn limbs are still strewn all over the site," one reporter said on Sunday.

Police called in extra contingents to reinforce security as panicked relatives crowded the local hospital.

No individual or group has yet claimed responsibility, but authorities have vowed to hunt down the criminals.

"The culprits must be found and punished," Home Minister Mohammad Nasim said on a visit to the blast site.

Police said Awami League lawmaker Shamim Osman, who was holding a meeting of party workers and supporters at the time of the blast around 10 p.m. (1600 GMT), was injured but recovering in hospital.

Witnesses said a brief exchange of gunfire followed Saturday night’s blast. "Apparently some people were trying to shot Shamim Osman while he was being transferred to hospital. His supporters returned the fire," one said. Police declined to comment.