Christ Church Anglican Cathedral was among the target of bomb attacks in Zanzibar this week.

The bomb went off at the main entrance to the cathedral in Stone Town at around 1pm local time on Monday. Eyewitnesses said the bomb was detonated remotely.

This was followed by a second bomb explosion at Mercury's, a well-known restaurant named after Freddie Mercury, who was born in Zanzibar.

The attacks came one day after a church belonging to the Evangelistic Assemblies of God Tanzania was bombed as the service was coming to a close.

Christian Solidarity Worldwide reports that the number of injured and the extent of the damage remain unclear.

The semi-autonomous status of Zanzibar and its relationship with the mainland is a key issue in Tanzania's forthcoming elections, and a separatist group called Uamsho (Awakening) has been campaigning for Zanzibar to become a fully independent nation.

Recent years have seen an increase in violence against church leaders, starting with an attack on a Catholic priest by unknown gunmen in December 2012. The priest was wounded but survived the attack.

Last year in February, however, attackers murdered a Protestant pastor and a Catholic priest, and in September a retired Catholic priest was seriously injured in an acid attack.

Then last month, a mob burst into the Sunday service of a Pentecostal Evangelism Fellowship of Africa in Kisauni in an attempt to kill the pastor. When they were unable to find him, they beat up a visiting clergyman.

CSW reports that local Christians have received threatening text messages, and that leaflets have been distributed giving the names of church leaders earmarked for assassination.

At least 20 churches have been looted, set on fire, or demolished by mobs, and police have failed to bring the perpetrators to justice.

CSW Chief Executive Mervyn Thomas said the use of a remote device during Monday's bombing indicated that perpetrators of religious violence in Zanzibar were "attaining new levels of sophistication and planning".

"The Government of Tanzania must undertake swift investigations in order to ensure that those responsible for these attacks are brought to justice," he said.

"It is also vital for the Tanzanian Government to uphold freedom of religion or belief for all its citizens, including the Christian minority in Zanzibar, in line with its international obligations under article 18 of the International Covenant on Civil and Political Rights."