TEHRAN, Iran (CNN) -- Three leading Iranian reformists who have rejected the results of last month's election questioned the legitimacy of President Mahmoud Ahmadinejad's government Wednesday.



Mehdi Karrubi is vowing to "stand by the people and the revolution, until the end of my life."



This comes as Ahmadinejad is set to take office at the end of the month.



Presidential candidate Mehdi Karrubi wrote a letter in his party's newspaper, saying he would not recognize the government and vowing to "stand by the people and the revolution, until the end of my life."



His statement prompted Iran's government to block publication of the newspaper. Read Karrubi's letter PDF



Ahmadinejad's main political rival, Mir Hossein Moussavi, also released a statement Wednesday criticizing the government and its crackdown on the media, which he said has created a "bitter, coup d'etat atmosphere" in Iran.



"We will stand firmly in order to preserve this valuable accomplishment [revolution]," Moussavi said. "Unless we succeed in this, this government will not have legitimacy. The system and the heritage of the Islamic revolution are the fruits of our 200-year-old struggle against oppression."Read Moussavi's statement PDF



Iran's former reformist President Mohammad Khatami called on Iranians to keep up the struggle, noting that "all doors are not yet closed." Read Khatami's letter PDF



"We must not lose our social capital this easily," Khatami told progressive Iranian newspaper Tahlile Rooz. "I know Moussavi as one of the faithful, original and valuable capitals of our revolution, and considered his return to the political scene as a great chance."



In a separate posting on his Web site, Khatami accused the government of having restricted communications in the country. "The state-owned media outlet shows the same scenes over and over again, in order to provoke people's feelings," he said. "Where was this media outlet when tens of people were killed and hundreds of people beaten?"



In his statement, Moussavi called for the release of jailed reformists and said he will participate in the creation of a "legal organization" that will release proof of fraud in the June 12 presidential election and take its case to the courts.



He said the current political issue is a "family dispute" and cautioned against asking for outside help, warning, "We will regret it."



"Many" have asked Moussavi to end his struggle and "close my eyes," but he warned, "If we do not stand our grounds now, then we will have no guarantees that we won't be at this exact point in the future, face to face with the bitter events of this election."



Karrubi called the actions of the government before and after the controversial June 12 voting "the foundation for the annulment of the elections," according to a copy of his letter on the party newspaper's Web site.



"I will not recognize the legitimacy of the government which has resulted from this process," Karrubi said in the letter.



The 72-year-old cleric also said he "will not participate in any of its processes, in any way" and said he is "ready to cooperate with pro-change people and groups."



Karrubi's party, Etemed Melli, said Iran's Ministry of Culture and its attorney general prevented the publication of its newspaper because it carried the letter.



He and Moussavi have questioned the legitimacy of the vote count of the presidential election that gave Ahmadinejad an overwhelming victory. That outrage sparked bloody street protests and a clampdown on international media coverage, as well as access to certain Web sites.



At least 20 protesters were killed in the chaos and more than 1,000 were detained in Tehran, the head of Iranian internal security forces Brig. Esmaeil Ahmadi said, according to Iranian state-run media reports on Wednesday.



The actual death toll may be higher, but restrictions on media have made it difficult to verify.



Human Rights Watch on Wednesday called on Iran to release prominent Iranian reformist Saeed Hajjarian, who has been imprisoned since June 15. He
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