TEHRAN, Iran - Iranian opposition leader Mir Hossein Mousavi vowed a "year of perseverance" in his fight against the government, in an Internet message on Friday marking the Persian new year.

Mousavi, who remains steadfast in rejecting the re-election of President Mahmoud Ahmadinejad in June last year, said a "retreat would be treason to Islam, the nation and to the blood of the martyrs."

"We will face issues and problems in (the Iranian year to March 2011)," Mousavi said in a statement on his website. He said some of the issues are connected to popular demand after the election, "which is a rightful demand."

"These demands will continue and the new year will be the year of perseverance for this rightful and legal demand."

Once seen as a pillar of the Islamic regime, former premier Mousavi is now a bitter critic of the country's hardline leadership, along with fellow opposition leader Mehdi Karroubi.

Iran witnessed one of its worst political crisis when Mousavi, Karroubi and hundreds of thousands of their supporters took to the streets after Ahmadinejad's re-election, protesting that the poll was massively rigged.

The Persian New Year, or Nowrouz, starts on March 21 and marks the arrival of spring.

For his part, former reformist president Mohammad Khatami, a strong supporter of the opposition movement, called for electoral changes and the release of political prisoners.

"We should learn our lesson from past events and choose a different path," the website Parlemannews.ir quoted Khatami as saying at talks with a group of reformists.

"Several issues can be solved if prisoners are freed, political movements enjoy legal freedom, criticism is allowed within legal boundaries (and) the ground is prepared for healthy and free elections."

Mousavi also lashed out at the government's handling of the inflation-hit economy, predicting the year ahead would see unemployment and poverty mount as investment falls.

"The adventurist policies, which lack wisdom, have created a threatening situation for us," he said.

"We are in the worst situation when it comes to foreign policy and international relations, and we will witness more sanctions and pressure."

In recent weeks, Washington has stepped up efforts to garner international support for a fourth round of UN sanctions against Tehran for doggedly pursuing its nuclear programme.

Mousavi called for people to have the freedom to "show their thoughts which would be Islamic and help in the progress of the country."

"We should not be scared and turn our back to the demand of the nation."

Meanwhile, hardline cleric Ahmad Khatmani, who has repeatedly lashed out at the opposition movement, urged people during his Friday prayer sermon to remain "alert" in the light of events that occurred since the election.

"Last year's events were complex, but there were lessons to be leaned. The most important lesson was that we must be alert," the cleric said at Tehran university.

He also reiterated that it was the "presence of people" that "spoiled the plots of enemies and their lackeys."

Since the election crisis erupted, Iranian hardliners have criticised opposition supporters of planning to topple the Islamic regime with the help of foreign powers.