Leading opposition candidate Mr Mousavi accused Iran‘s nationalised telecommunications provider of deliberately shutting down the system.
He also alleged that some of his representatives were barred from entering polling stations to monitor the vote.
Mr Mousavi criticised Mr Ahmadinejad’s handling of the struggling economy during the campaign and ridiculed the president’s hyperbolic rhetoric on international affairs.
But Mr Ahmadinejad insisted that the economy has fared better since he has been in power and accused his rivals of corruption.
The communist Tudeh Party of Iran was critical of all candidates, but called for maximum participation to vote against the reactionary Mr Ahmadinejad and and fundamentalist Mr Rezaei.
The Tudeh Party said that four years of Mr Ahmadinejad’s presidency had seen detrimental economical policies leading to bankruptcy of the manufacturing sector along with increasingly high unemployment and inflation.
It also criticised unprecedented waste and misuse of oil revenues, a heightened atmosphere of suppression and terror and ongoing attacks on the working class, women’s, youth and students’ movements, and intensified pressure on religious and national minorities.
It said that divisions among reformers before the 2005 presidential election, coupled with a mass boycott by voters, had helped Mr Ahmadinejad into power.
But the Tudeh Party said: “Existing signs indicate the general will of the people to participate in the elections and to free themselves from Ahmadinejad’s government.
“The will of the people must be converted to a broad social force going to polls.”