More than 100 Iranian Christians were arrested last week in another sign of increasing pressure on Iran's believers.

According to Open Doors, most were allowed to return home after a few hours but were told to expect a call from the Ministry of Intelligence. All of them had their mobile phones confiscated.

People who were suspected to be the leaders of the groups of Christians were held in detention.

Open Doors UK's head of advocacy, Zoe Smith, said long prison sentences have been given to Christians who refused to leave Iran after their previous arrests. However, long prison terms are now becoming common for Christians who have been arrested for the first time.

;This spike in arrests is highly concerning. It follows an established trend of the Iranian government – as the number of converts to Christianity increase, so the authorities place greater restrictions on churches,' she said. 'The restrictions are worse for churches seen to be attended by Christians who have converted from Islam. Not only that, but the government is asking unreasonably high bail amounts and seeing longer prison terms for Christians.'

House churches for Christians from Muslim backgrounds have been raided and leaders given long prison sentences. Consequently, many converts have fled abroad or practise their faith in isolation. Christians from the government approved historical Armenian and Assyrian churches who reach out to Muslims have reported discrimination, harassment, physical abuse and imprisonment.

Smith continued: 'Some Christians disappear from their communities after serving a sentence; church leaders are put under pressure to leave the country or face an arrest; house churches weaken as their members choose to decrease their meeting hours and minimise their activities; some Christians lose the contact with their churches altogether becoming isolated.'

Iran is number 10 on the Open Doors World Watch List ranking the 50 countries where it is most difficult to be a Christian.