The rights group said the amputation, carried out at a prison in the northern province of Mazandaran, was "an abhorrent form of torture".

Iranian officials said the man was found guilty of 28 cases of theft.

Iran's Islamic penal code says theft "on the first occasion" is punishable by the amputation of four fingers of the right hand.

Iranian authorities have defended amputation as the best way to deter theft despite protests by international human rights organisations.

However, reports of such punishments are rare.

Saleh Higazi, Amnesty's deputy director for the Middle East and North Africa, said in a statement on Thursday that the premeditated "maiming and mutilation of individuals is not justice".

"It is a harrowing assault against human dignity. Reforms to Iran's penal code that would put an end to this outrageous practice are long overdue."

The Iranian judiciary's news agency Mizan said the sentence had been carried out on Wednesday in the capital of Mazandaran, Sari. The convicted man was not named.

Iran is an Islamic republic and its legal system is based on a strict reading of Sharia law.

In January 2018 authorities in north-eastern Iran amputated the hand of a 34-year-old man convicted of stealing sheep.

Similar punishments have been meted out in Saudi Arabia, Nigeria and Somalia.

Iranian authorities were widely criticised by rights groups last year for flogging a man who was convicted of consuming alcohol as a teenager more than 10 years earlier.

In February this year the US and human rights groups expressed anger after Iran executed two teenage boys for rape.

Amnesty said at the time that Iran had executed nearly 100 juvenile offenders since 1990 - the highest number of any nation.