Tens of thousands of worshippers have been flooding into Iraq's holy city of Karbala despite the Covid-19 pandemic for the Shia Muslim pilgrimage of Arbaeen, one of the world's biggest religious gatherings.

Arbaeen marks the end of the 40-day mourning period for the seventh century killing of Imam Hussein, Prophet Mohammed's grandson, by followers of Caliph Yazid, and falls on Thursday.

His martyrdom was mourned this year at the end of August during Ashura ceremonies in Iraq and around the world, but unlike previous years, Iraq had closed its borders to non-residents, allowing only residents to take part in Ashura.

Iraq's borders opened for Arbaeen but with restrictions on Arbaeen arrivals, in a bid to stem the spread of Covid-19.

The virus has hit Iraq hard, with more than 375,000 people infected and almost 9,500 deaths.

In 2019, an estimated 14 million Shia pilgrims flooded Iraq to attend Arbaeen, including about two million from neighboring Iran.

This year, however, only 1,500 pilgrims per country are being allowed to fly into Iraq.

"Every day, between six and 10 planes land and more are coming in the days ahead," said Issa al-Shemmari, airport director in Najaf, another holy city south of Karbala.

In keeping with tradition, Iraqis have been walking to Karbala for Arbaeen from all over the country.

On roads to the city, "mawakib" service tents have been erected to provide pilgrims with free food, drink and accommodation.