A huge crowd flooded central Tehran in a sea of red and black on Wednesday to mourn a soldier who has become a symbol of the fight against the Islamic State group.

Last month's capture and beheading of Mohsen Hojaji, a 25-year-old volunteer with the Revolutionary Guards, has triggered a rare outpouring of public support for Iran's military campaigns in Syria and Iraq. 

The stoic image of Hojaji standing with a brutal-looking IS militant holding a knife to his back went viral last month, and his memorial service filled Imam Hossein Square in Tehran and jammed the streets all around. 

Supreme leader Ayatollah Ali Khamenei met Hojaji's relatives ahead of the service, saying: "There are many martyrs and all are dear before God Almighty, but there is something special about this young man."

The service gained added significance as it fell in the 10-day period of Moharram when Iranians mark the killing in the seventh century of Imam Hossein, one of the most revered figures in Shiite Islam. 

Iran has couched its support for Syrian President Bashar al-Assad in religious terms, calling its fighters "defenders of the shrines" in reference to Shiite monuments in Syria. 

The crowd in Tehran wore almost exclusively black and waved red flags to symbolise the blood of martyrs.

Hugely popular singer Sadegh Ahangaran delivered an emotional lament of "We are defenders of the shrine until doomsday. Hey, mad knife-wielders, this house is full of Hojajis."

Hojaji was captured after IS launched an attack on a Revolutionary Guards outpost near Al-Tanaf in Syria on August 7. 

His dismembered body was later delivered to Lebanese militant group Hezbollah, an Iran ally, as part of a ceasefire deal, Iran's Tasnim news agency reported.

An AFP photographer said the square was already full by early morning, before the Revolutionary Guards barred all foreign media from covering the event. 

With Iran's clerical leadership united behind the military campaigns, the issue barely featured in May's presidential election and it has been hard to gauge the true extent of popular support for the conflict.

Iran faces a direct threat from the Islamic State group -- which considers Shiites to be apostates who must be wiped out. 

But critics say Iran has propped up Assad's murderous regime and is seeking to expand its reach across the region. 

Casualty figures are rarely divulged. The most recent toll, given in March by the head of the veterans organisation, said 2,100 fighters had died in Syria and Iraq, although he did not specify how many were from "volunteer" brigades recruited in Afghanistan and Pakistan.