Four Christians imprisoned in Iran have been released on bail after being held for four months in a notorious jail.

Middle East Concern reports that three of the men, Eldar Gurbanov (48), Yusif Farhadov (51) and Az (37), were from from Baku in Azerbaijan, and had travelled to Iran in June for a wedding.

Security agents raided the wedding party on June 24, and arrested the three along with Naser, an Iranian Christian convert, and six others.

While the six were released, the four men were taken to Iran's notorious Evin prison for interrogation. They face charges of engaging in missionary activity and propaganda against the regime.

For two months, they were kept in solitary confinement and were refused access to legal counsel or consular assistance.

In September they were transferred to shared cells, and were finally released on bail on October 29.

On Sunday, the three Azerbaijanis were allowed to travel back home. Middle East Concern said campaigners remain hopeful they will be acquitted of all charges.

Evin prison was where US-Iranian Christian convert Saeed Abedini was held for years on charges of undermining state security. He was released in January this year as part of a prisoner swap.

Ranked ninth on Open Doors' list of countries where it's most dangerous to be a Christian, open churches are forbidden in Iran and converting from Islam – the state religion – is punishable by death for men, and life imprisonment for women. Last year more than 100 Christians were arrested or imprisoned and allegations of torture have emerged.

However, it is also considered to have the fastest growing evangelical population in the world.