According to one of al-Sistani's communications officers, security guards arrested a man on Thursday as he tried to break into the Ayat Allah's office in Najaf, 160km south of Baghdad, to carry out an unspecified "criminal act".
Shaikh Abd al-Mahdi al-Karbalai, one of the Ayat Allah's representatives, said on Friday in an interview with Lebanon's al-Manar television that "arresting someone does not mean that there was an assassination attempt".
Al-Manar TV, owned by the Lebanese Shia party Hizb Allah, also quoted al-Sistani's son, Muhammad Rida, as saying reports of an assassination bid were "inaccurate".
Hamid Khffaf, al-Sistani's aide in Beirut, told Aljazeera that the cleric was not targeted on Thursday.
"There was no assassination attempt targeting al-Sistani," he said.
Khaffaf confirmed that security had been tightened in the last two weeks to protect al-Sistani, as they had received warnings of possible attacks. "But I deny the reports saying al-Sistani was attacked," he added.
"Arresting someone does not mean that there was an assassination attempt"
Shaikh Abd al-Mahdi al-Karbalai,
Shia politician Muwaffaq al-Rubaie said on Friday that the cleric, 73, was safe and sound. "What I can say is that Sayyed al-Sistani is is good health, he is unharmed," said Rubaie, who sits on the Iraqi interim Governing Council.
But he would give no details and refused to confirm or deny reports that al-Sistani was the target of an assassination attempt, although he was the first to have reported that there has such an attempt on Thursday after he met the cleric in Najaf.
Several other members of the Governing Council cast doubts on media reports that gunmen shot at al-Sistani as he left his office for home.
Sunni member Samir Sumeidai pointed out that al-Sistani lives and works in the same compound. "I doubt this report is true," he said.
The US-led occupation forces on Friday said it was still looking into the reports.