Moderate cleric Mehdi Karroubi became the first candidate to officially declare his nomination Sunday for next June's presidential elections to challenge Iranian President Mahmoud Ahmadinejad.

The 71-year-old cleric is head of the moderate opposition party Etemad Melli (National Trust) and the party's choice run in the June 12 elections.

"It will be quite difficult but I am still ready to take the challenge for changing the political status quo," Karroubi said in a press conference in Tehran.

The cleric referred to the president's economic and foreign policies as the main problems of the current administration which should be amended.

On the controversy over Iran's nuclear programmes, Karroubi said that Iran should insist on its internationally acknowledged right to pursue a civil nuclear programme but at the same also pay attention to Western concern and guarantee the peaceful nature of the nuclear projects.

Karroubi served twice as parliament speaker, during 1990-92 and 2000-04. In the 2005 presidential elections, he missed reaching the second round by a few votes.

Karroubi and his Etemad Melli party, which also runs a newspaper under the same name, have in recent years distanced themselves from reformists close to former president Mohammad Khatami, whom they feel has drifted away from the Islamic system that has ruled the country since the 1979 revolution.

Khatami has not yet declared his intention to run in the next presidential election, but many analysts expect him to.

Another potential moderate candidate is cleric Hassan Rowhani, who served under Khatami as Iran's chief nuclear negotiator.

The failure of reformists to form a united front increases Ahmadinejad's chances of being re-elected, although the president is under fire for Iran's high inflation, which has hurt many people who voted for him in 2005.

"We are a party and have our own framework and accordingly appoint an independent candidate as well," Karroubi said.

It was not yet clear whether Ahmadinejad would obtain unanimous support of the conservative factions or if neo-conservatives would name their own candidate, as they did in the March parliamentary elections.

Ahmadinejad is also said to enjoy the support of Supreme Leader Ayatollah Ali Khamenei who, according to the constitution, has the final say in all state affairs. But some of the ayatollah's advisers, including former foreign minister Ali-Akbar Velayati, have openly criticized the president's policies.

While a low turnout would be seen as favourable to Ahmadinejad, a high one could indicate more voters coming to the polls who are against the hardline policies of the president and want a change in the country's political climate, analysts said.