The incident took place at around 9am, when a white MPV crashed into the gate of the police headquarters.
Indonesia is on high alert after a wave of recent bombings in the country's second-largest city, Surabaya.
While the police managed to shoot four of the attackers, the remaining five escaped, said the police chief of Riau.
"Four have been shot to death and one fled", said spokesman Setyo Wasisto.
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TV footage showed one man lying on the ground with a long sword next to his body and an armoured vehicle stationed outside the police station.
Earlier, an internal police report said that one of the dead men had a suspected bomb strapped to his body.
We'll bring more updates for you from the Riau Provincial Police HQ attack as the story develops.
The perpetrators have not been identified or a motive determined.
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SUSPECTED militants carrying swords and a bomb have attacked a police station on the Indonesian island of Sumatra, as Islamists continue to wage war against the south-east Asian nation.
Islamic State claimed responsibility for an attack on Wednesday in Sumatra, Indonesia that killed one policeman and wounded two, the group's Amaq news agency said.
The attack came days after a series of terrorist strikes in the country, including suicide bombings in three churches on Sunday in Surabaya city, the provincial capital of East Java and a suicide bombing at the police headquarters in the city, which left at least 25 people dead and dozens injured.
Police suspect they were carried out by a cell of the Islamic State-inspired group Jemaah Ansharut Daulah (JAD), an umbrella organisation on a U.S. State Department terrorist list that is reckoned to have drawn hundreds of Indonesian sympathisers of Islamic State.
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