Barred from Mecca amid an escalating spat between Tehran and Saudi Arabia, masses of Iranian Shiite faithful have converged on the holy Iraqi city of Karbala for an alternative pilgrimage.

The row that has prevented Iranians taking part in this year's hajj pilgrimage is diverting hundreds of thousands to the shrine of Imam Hussein, one of the holiest sites in Shiite Islam.

"I expect the number of pilgrims to reach a million, about 75 percent of them Iranians," Adel al-Mussawi, a shrine official, told AFP.

Not all of those had planned to travel to Mecca but many of the 64,000 Iranians who were allocated places for this year's hajj ended up in the holy Iraqi city this weekend.

Visiting the Imam Hussein shrine does not have the same religious significance as the hajj, which is a pillar of Islam and therefore an obligation for Muslims who are able at least once in their lifetime.

But followers of the Shiite sect of Islam feel more at home in Karbala than in Mecca, where around 2,300 people died in a stampede last year, according to an AFP tally, including 464 Iranians.

"Karbala is normal for us. We always come here. This year they have blocked the path (to Mecca) and no one can go," said Shukrullah, a white-haired Iranian pilgrim sitting on a rug near one of the gates to the mausoleum.

"It's our duty to come here. This is an Islamic country. It's good," he said.

Iran has accused Riyadh of incompetence and of failing to investigate the 2015 disaster or take satisfactory precautions for this year's pilgrimage.

Talks broke down between the two regional powerhouses and Iranians were denied entry.

A war of words has since escalated, with both countries' top clerics exchanging sharp words -- Iran's Ali Khamenei calling Saudi monarchs a "cursed, evil family" and Saudi's Abdulaziz al-Sheikh saying Iranians were not real Muslims.

"The Saudi-Iranian conflict has forced Iranians to come to Karbala to visit the shrine of Imam Hussein," Mussawi said, adding: "For the Shiites, this is worth 70 hajj."

For the city, which lies about 80 kilometres (50 miles) southwest of Baghdad, the extra influx of pilgrims is nothing out of the ordinary.

- 10-year wait -

"We have prepared transport, accommodation and security. We are used to handling bigger occasion such as Arbaeen so we can handle this," Karbala Governor Aqeel al-Turaihi told AFP.

In the Friday sermon read by his representative Abdul Mahdi al-Karbalai, Iraq's top Shiite cleric Ali al-Sistani appealed for respect and tolerance among all Muslims.

Yet resentment ran deep in the ranks of the Iranian faithful who were barred from Mecca, where the hajj got under way on Saturday.

"Last year, how many people were killed from all over the world? They (Saudi Arabia) killed all of them, but no one did anything to them," said Shukrullah, sheltering from the midday sun with his family near lockers where the faithful leave their shoes before entering the mausoleum.

Unlike Shukrullah, Nasirah, a woman from the Iranian city of Ahvaz, has not yet performed the hajj and predicted that the substitution trip to Karbala could become a habit.

"In Iran, the pilgrims... pay to get a visa and go to hajj. We in Iran wait a long time to get a chance to go. It can take 10 or 15 years," she said.

"So I said let's go for Arafah day in Karbala," Nasirah said, referring to a prayer performed by Shiites in Saudi Arabia's Arafat plain on the second day of hajj.

"If we are in Karbala, it's the house of God, it can be considered hajj for us. So for the next few years, we will be coming to Karbala -- what can we do?"