Joel Magarey – West Papuans who attended a church meeting to pray for Australian activists sailing towards West Papua in a "Freedom Flotilla" have been arrested by Indonesian police, Amnesty International has established.

Indonesian naval vessels have been deployed along the West Papuan border in a "show of force" aimed at the flotilla and are stopping local boats from leaving the flotilla's destination port, the Australian activists say.

Amnesty International last night criticised the Indonesian government for arresting the four "peaceful activists" – Apolos Sewa, Yohanes Goram, Amandus Mirino and Semuel Klasjok – and called for their "immediate and unconditional release".

Amnesty International's Indonesian representative, Josef Benedict, said the four men had been arrested on Wednesday after taking part in a "peaceful prayer ceremony at the Maranatha church in Sorong city in solidarity with the visit of the Freedom Flotilla to the Papuan region".

"Amnesty International believes they have been arrested and detained solely for their peaceful political activism and calls for their immediate and unconditional release."

An apparently local report published on the flotilla's website indicated the arrests had occurred after some participants in the prayer ceremony had attempted to hold a press conference outside the church.

Mr Benedict said "credible local sources" had informed him that the Morning Star – the banned West Papuan independence flag – had been raised during the ceremony.

Fifteen Aboriginal, Papuan and other Australian activists are attempting to sail to West Papua in two yachts in a risky venture aimed at supporting West Papuan independence. An Indonesian embassy spokesman said Indonesian police would conduct appropriate investigations.