The blast occurred in front of a police station in Jorhat, 310 kilometres (192 miles) east of Assam's main city of Guwahati.

"The bomb was concealed in a sack and kept on a bicycle parked near the police station," Jorhat police chief Deepak Choudhury told AFP by telephone.

The injured, four of them policemen, were moved to a local hospital.

"The area was crowded with people when the blast took place. It was a very powerful explosion that was heard from a distance of about two miles," another police official said.

It is the fourth explosion in two days in the area. On Sunday, 10 people were wounded in three separate blasts in Assam, with police blaming all the attacks on the outlawed United Liberation Front of Asom (ULFA).

Such attacks are common each year in Assam in the run-up to India's Independence Day on August 15.

The ULFA, fighting for an independent homeland since 1979, usually target crude oil pipelines, trains and road and rail bridges, besides federal soldiers.

The rebels have for years been boycotting Independence Day and Republic Day celebrations to protest New Delhi's rule over the vast region rich in oil, tea and timber.

India's seven northeastern states are riven by a myriad of insurgencies with more than 30 rebel armies operating there with demands including secession, greater autonomy and the right to self-determination.