TEHRAN (AFP) - Irans former president Mohammad Khatami on Saturday openly threw his weight behind ex-PM Mir Hossein Mousavi, who is contesting the June 12 presidential election. Speaking to thousands of supporters at a rally in Tehrans indoor Azadi sports stadium, Khatami said: Stand up and do not miss this rare opportunity. All of you, come to the ballot boxes and by writing down the name of Mousavi, fulfill your duty to the revolution and to Islam and also define your own fate, the charismatic cleric said. To a cheering crowd of men and women, many dressed in Mousavis trademark colour green or wearing green scarves or wristbands, Khatami said Iran was facing several restrictions. I know that filtering of thoughts and restriction of freedom has increased ... Interference in electoral matters ahead of the polls and at the time of the polls has also increased in order to prevent the will of the people from making itself heard, he said. But I know that the nation has become more experienced and alert, said the ex-president, who was elected to two consecutive terms. Even as he spoke, the Ilna news agency reported that Iran had blocked access to Facebook ahead of the polls, allegedly to prevent supporters of Mousavi from using the site for his campaign. According to certain Internet surfers, the site was banned because supporters of Mir Hossein Mousavi were using Facebook to better disseminate the candidates positions, Ilna considered close to reformists said. As a symbolic gesture, the Saturdays pro-Mousavi rally was held on the anniversary of Khatamis first election victory in 1997 on May 23 the day dubbed as the day of reform movement in the Islamic republic. Criticising the policies of President Mahmoud Ahmadinejad, Khatami called for opening the door of Iran to the world and the doors of the world to Iran. During his presidential term, Khatami earned the tag reformist for several liberal initiatives that still continue under hardliner Ahmadinejad. But despite his best efforts as president, several critics say Khatami was not firm enough against establishments in the Islamic republic, leaving his mostly youthful supporters disillusioned and disappointed. Saturdays rally, although conspicuous by Mousavis absence as he was addressing another rally in Isfahan, saw thousands of enthusiastic young supporters vowing to vote for the ex-premier who is credited for his successful management of the Iranian economy during the war with Saddam Hussein in the 1980s. He will support human rights and offer social freedom, said Maryam, 21, a student as she shouted pro-Mousavi slogans along with thousands of others in the packed stadium.