Thousands of protesters have gathered in a central Tehran square on the anniversary of the death of the first Shia Imam to demonstrate against damage to a gravesite in Damascus considered a shrine to the Islamic sect.

The demonstrators beat their chests and chanted slogans against Syrian Sunni rebels, accusing them of desecrating the burial site of Zainab, the granddaughter of the Prophet Muhammad.

The complex, which includes the Sayyidah Zainab Mosque that contains the grave of Zainab, was damaged in fighting between rebels and Syrian government troops.

The rally, which occured as Iranians commemorated the anniversary of the death of Ali ibn Abi Talib, who is considered the first leader of Shia Muslims, came after Iranian media reported that Syrian rebels desecrated the shrine with mortar fire and killed its caretaker on July 19.

"We have gathered here to protest against the desecration of the shrine of Hazrate Zeinab so that Wahabis and Salafists and their supporters understand that we defend our holy shrines," said Mohammad Reza Mohsenzadeh, a protester in Iran.

The UK-based Syrian Observatory for Human Rights, which relies on activists on the ground, said the shells struck the edge of the shrine's complex and caused only minor damage to its external wall.

The burial site of Zainab is also revered by Sunnis.