Hundreds of edgy young men from al-Sadr's army, carrying sniper and assault rifles and rocket-propelled grenade launchers, were seen on Saturday positioned in the area around the mausoleum of Imam Ali, the city's holiest shrine.
Many more were seen on building rooftops and in alleys off the four main streets leading to the mausoleum.
Most of al-Sadr's fighters are from poor Shia families from outside Najaf province, mainly from his stronghold of al-Sadr City in Baghdad.
"I am here to die for Islam and al-Sadr because I have chosen a worthier life up there in paradise," said a 16-year-old boy from Baghdad.
He was carrying an assault rifle and wearing a green ammunition belt marked with the names of al-Sadr and the revered Shia imams, Ali and Husayn.
Al-Sadr, who the occupation forces are looking to capture in connection with the murder of a rival cleric last year, appeared to be bolstering his force inside Najaf over the past few days with more young recruits, or "volunteers" as he calls them, from Baghdad and southern cities like Amara, Basra and al-Nasiriya.
"I am here to die for Islam and al-Sadr because I have chosen a worthier life up there in paradise"
fighter, Mahdi army
The stepped-up presence of al-Sadr followers has visibly worried other armed Shia groups in the city such as the Badr Organisation, which is part of the Supreme Council for the Islamic Revolution in Iraq (SCIRI), represented on the Iraqi Governing Council.
Even guards entrusted with protecting Grand Ayat Allah Ali al-Sistani, Shia Islam's most eminent cleric, said they feared "knee-jerk reactions" by Mahdi fighters against al-Sistani's office and base on Rasul Street off the mausoleum.
"I hear many Mahdi fighters grumbling that al-Sistani is just sitting inside his air-conditioned home saying nothing to support their fight against the Americans," said the guard on Friday.
Al-Sistani has called for a peaceful resolution of the standoff with al-Sadr and has called for the respect of the sanctity of Najaf, and has steered away from any statement that might be perceived as a call for jihad against the US occupation or an endorsement of al-Sadr.
About 2500 US soldiers are camped in a desert area known as Bahr al-Najaf, west of the city, and a unit from this contingent has moved into a base between Najaf and Kufa, to the north, that was previously occupied by Spanish and Latin American troops, who have withdrawn.