The Council of Guardians, the conservatives-controlled body that vets all elections candidates in the Islamic Republic, had rejected all 55 reformists affiliated candidates.

Polling started at 08:00 (0430 GMT) at 930 stations with 15,000 people tasked with administration, supervision or establishment of security, the official news agency IRNA reported.

Leading reformist deputies had warned that they might be forced to boycott the polls if the Guardians do not change their decision.

However, the by-election went ahead according to schedule despite the Majles bid to postpone it since the powerful Guardians had quashed its efforts for a set of amendments to the electoral law.

Officials said that turn out was heavy and therefore they extended the closing time for two hours, but eyewitnesses told Iran Press Service that voters were much lower than all the previous elections.

Early exits are expected as early as Saturday, the officials said.

"That the elections were extended for 2 hours was adding insult to injury", one source in Gorgan said, claiming that many polling stations were empty as in the afternoon.

Mashallah Shamsolva’ezin, a prominent Iranian journalist said the conservatives have turned the elections into a "selection" of their own choice.

"If the conservatives think by adopting such tactics they could replace the reformers in the eyes of the public and the society, they are absolutely wrong", Mr. Shamsolva’ezin told the Persian service of the BBC, expressing "deep concern" as the future of the country.

Though, according to its tradition, the Guardians did not provide any justification for the rejection of the rejected candidates, but informed sources said conservative’s sectarian might be one of the reasons, as the region is dominated by Turkmen, who, in their majority, are Sunni Muslims.

The Guardian Council further rejected parliament's proposal on the establishment of an arbitration body to settle legislative disputes between the legislature and the oversight council.