Shias all over the world mark the Chehlum or Arbaeen of Hazrat Imam Hussain (RA) 40 days after Ashura.

Chehlum processions kicked off in various cities of Pakistan under tight security, as cellphone services were suspended and traffic diverted from procession routes.

Millions of Shias also thronged to Karbala in Iraq for the culmination of one of the world's largest religious events.

The faithful have for days been streaming through the golden-domed mausoleum of Hazrat Imam Hussain (RA), the Prophet Muhammad's (PBUH) grandson who was killed in 680 AD by the armies of Yazid, despite the threat of militant attacks.

Many in the sea of black-clad devotees swarming the shrine walked days to reach Karbala, sometimes from cities as far afield as Basra, about 500 kilometres away by road.