Police in the district of Solo, 300 miles southeast of Jakarta, said on Friday that they had ordered churches to dig holes a metre deep outside their buildings. The instruction comes amid fears of Christmas bomb attacks and at the same time as a national clampdown on anonymous mobile phone use.
The Koran Tempo newspaper said police would search all 61 churches in Solo for bombs before services on Christmas Eve.
Raids were carried out on several houses in the Solo area after the triple suicide bombings on the resort island of Bali on 1 October, which killed 20 bystanders.
Azahari Husin, a Malaysian bomb-maker who was tracked down and killed by police in November, as well as Noordin Mohammad Top, a compatriot who remains at large, were linked to the bombings and are believed to have spent time in Solo.
In a separate security measure, all prepaid mobile phone users in Indonesia will have to register their numbers by April next year in an attempt to reduce fraud and deter terrorists.
Prepaid mobile phone users will lose their numbers if they fail to register by then, Gatot Dewa Broto, a spokesman for the director-general of post and telecommunications, said.
"It's also aimed at limiting the room for terrorists to communicate ... but we know the terrorists are smart," he said.
Several bombs in Indonesia are thought to have been detonated by mobile phone.
Last month Susilo Bambang Yudhoyono, the Indonesian president, ordered police to crack down on mobile phone users who sent racist text messages after a spate of them called for attacks on minority ethnic Chinese.
Indonesia's intelligence agency said last week that extremists may be planning attacks over Christmas and the New Year in large cities across the world's most populous Muslim nation.
Police in Jakarta have said that 18,000 officers will be deployed in the capital during this time.
Extra police will be deployed
over the Christmas period
Nineteen people were killed when Islamist extremists carried out a co-ordinated nationwide bombing campaign targeting churches and priests on Christmas Eve in 2000.
The attacks were blamed on Jemaah Islamiyah (JI), a network linked to al-Qaida which has also been blamed for a string of deadly attacks in Indonesia since then, including the 2002 Bali bombing, which left 202 people dead.