According to reports from journalists at the scene, who were covering another story right before the attack, a white Toyota Avanza smashed into the outer fence of the police headquarters.
The men had driven their vehicle into the police yard before getting out to stage the attack, Setyo Wasisto, national police spokesman, told a briefing.
Suddenly the militants exited the auto and immediately staged strikes with swords against policemen, said a police spokesman.
TV footage showed one man lying on the ground with a long sword next to his body and an armoured auto stationed outside the police station.
Editor's note: An earlier version of this article stated that at least three men had attacked the Riau Police headquarters.
Local media said one attacker may have had a bomb strapped to his body but police have not confirmed the reports.
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The perpetrators have not been identified or a motive determined.
It was not clear if the latest incident was linked to other attacks this week, which saw two families - who all belonged to the same religious study group - stage suicide bombings at churches and a police station in Surabaya on Java island, Indonesia's second biggest city.
Just hours after the church bombings, a further three people in another family were killed and two wounded when another bomb exploded at an apartment complex about 30km from Surabaya.
Police suspect they were carried out by a cell of the Islamic State-inspired group Jemaah Ansharut Daulah (JAD), an umbrella organisation on a US State Department terrorist list that has allegedly drawn hundreds of Indonesian sympathisers of Islamic State.
Indonesia is still reeling from the shock of a series of suicide bombings in recent days.