TEHRAN, Iran - Iranian opposition leader Mir Hossein Mousavi vowed not to let the blood of protesters killed in postelection crackdown go in vain as he met with the family of a young man shot to death during the turmoil, reformist Web site reported Wednesday.

Mousavi, meanwhile, announced Wednesday that he will attend Tehran's main Friday prayer services this week for the first time, a key symbolic assertion of the opposition's presence after the crackdown.

The sermon Friday is due to be delivered by Akbar Hashemi Rafsanjani, a powerful cleric in Iran's leadership who has not performed the sermon since the turmoil erupted following the disputed June 12 presidential election. Rafsanjani is a top rival of President Mahmoud Ahmadinejad and is believed to be a strong behind-the-scenes backer of Mousavi.

"I'll join you (for prayers) on Friday July 17," Mousavi said, according to his Web site ghalamnews.ir.

The main Friday prayer sermon at Tehran University is a significant political platform that hard-line clerics have used in recent weeks to demand a halt to postelection protests and spread the message that that turmoil was fueled by foreign enemies. Rafsanjani's sermon could give the first opposition voice in the sermon.

The pro-Mousavi Web site mowjcamp.com said reformist leaders will hold street protests after attending the Friday prayers.

Thursday night, several hundred supporters gathered, some chanting "death to the dictator," as Mousavi and his wife visited the family of Sohrab Aarabi, 19, who disappeared during a June 15 protest, according to mowjcamp.com. Footage from the visit posted on the Web showed Mousavi moving through a crowd of well-wishers inside Aarabi's family home to his parents to express condolences.

Aarabi disappeared during a June 15 protest, and his family searched for weeks for news of his fate. They were finally notified on Saturday that he had been shot in the chest and died during the crackdown on postelection protests. Aarabi was buried Monday in the vast Behesht-e-Zahra cemetery on the outskirts of Tehran.

Mousavi and his popular wife Zahra Rahnavard made the visit as a top aide announced that Mousavi will create a new front grouping pro-reform political parties - the first concrete sign of what the opposition's next step will be to carry on its campaign against Ahmadinejad's government.

Mousavi claims to have won the June 12 election and that Ahmadinejad's victory in official results was fraudulent. But Iran's supreme leader, Ayatollah Ali Khamenei, has backed Ahmadinejad, and security forces crushed street protests by hundreds of thousands of Iranians in support of Mousavi.

"We won't let the blood of these youth go in vain," Mousavi told Aarabi's family during the visit, according to the Web site norooznews.org.

Aarabi's mother, Parvin Fahimi, said she would take the case of her slain son to domestic and, if necessary, international courts, the site reported.

Hundreds of candles were lit in the streets of the Tehran neighbourhood where the family lives, mowjcamp reported. Inside the home, the walls were hung with pictures of Aarabi wearing a green scarf over his shoulders - the colour of Mousavi's opposition movement.

Ahmadinejad is due to be sworn into office for a second term sometime between Aug. 2 and 6, the vice speaker of parliament Mohammad Reza Bahonar said Tuesday, according to the state news agency IRNA. The inauguration would take place a day after a ceremony in which Khamenei officially approves him as president.