TEHRAN- Iranian opposition politician Ebrahim Yazdi has been arrested while in hospital, an ally said on Thursday, the latest in dozens of detentions of pro-reformers since last week's disputed presidential election.

The ally, a member of the Freedom Movement, said Yazdi was taken away by security agents on Wednesday while he was in a Tehran hospital undergoing medical checks for stomach problems.

There was no immediate comment from the authorities.

Yazdi, who heads the banned Freedom Movement, was foreign minister in Iran's first government after the 1979 Islamic revolution that overthrew the U.S.-backed shah, but was sidelined as religious hardliners took control.

In an interview with Reuters on Monday, Yazdi said the June 12 election had revealed deepening establishment divisions and that the Islamic Republic faced its biggest crisis since the revolution three decades ago.

He also said seven other Freedom Movement members had been detained after last Friday's vote and warned of worsening political suppression in Iran.

Yazdi is an important opposition voice in Iran but has no influence on state policy and limited popular support.

Saturday's announcement of official election results showing hardline incumbent Mahmoud Ahmadinejad had won a landslide against moderate Mirhossein Mousavi sparked running street battles between Mousavi supporters and riot police.

Mousavi has accused the authorities of rigging Friday's vote. The government has dismissed the charge.

Earlier this week, reformist sources said police had detained over 100 reformers, including a brother of former President Mohammad Khatami and leading reformist Mohammad Ali Abtahi. Police denied Khatami's brother had been arrested.

On Wednesday, a pro-reform activist and a newspaper editor were also arrested, a reformist source told Reuters.

Scores of protesters have been detained in various cities over the last week, according to Iranian media.

Iran's Intelligence Ministry said it has arrested a number of main agents behind post-election violence in Tehran, the English-language Press TV said on its website on Thursday.

The Intelligence Ministry has identified and arrested a number of the main agents and elements behind acts of vandalism in recent days in Tehran, it quoted Intelligence Minister Gholamhossein Mohseni Ejei as saying.

(Reporting by Hossein Jaseb and Fredrik Dahl; editing by Jon Boyle)