Several masked men attacked the Riau Police headquarters at around 9 a.m. on Wednesday with sharp weapons and firearms, killing one police officer and injuring another.
Media said one attacker may have had a bomb strapped to his body but police did not immediately confirm the reports.
TV footage showed one man lying on the ground with a long sword next to his body.
Five people died in the attack, including the four suspected militants and a police officer who was hit by the vehicle.
No group has taken responsibility so far.
The police earlier disclosed that 8 militants with a white minivan launched the attacks.
Indonesian activists hold candles for the recent victims of terror attacks, during a vigil in Jakarta.
The report said that the men had driven their auto into the police yard before getting out to stage the attack. Media were at the station to cover a press conference about a drug bust when the attack happened.
Police shot a militant and arrested at least 13 others in a series of raids following the Surabaya attack.
The attack comes days after over two dozen people were killed in a series of terrorist strikes in Indonesia, including suicide bombings by a family of six in three churches on Sunday in the islands second largest city, Surabaya.
State news wires Antara said that two journalists who were at the headquarters were also wounded.