As Muslims around the world celebrate Eid al-Adha, people in Dhaka have been sharing pictures showing streets flooded with blood-stained water.

The blood comes from animals sacrificed for the festival, and had mixed with floodwater in parts of the Bangladeshi capital.

Why is this happening?

Nearly 100,000 livestock were sacrificed in Dhaka this year, according to BBC Bengali, mainly on the streets or at underground car parks in residential buildings.

At the same time, there was heavy monsoon rain for most of the day on Tuesday, flooding many parts of the city. Flooding has long been an issue in the older parts of the city with poor drainage systems.

In the Shantinagar suburb, and other areas, the blood and animal waste combined with the floods created these shocking scenes: residents wading through ankle-high bloody water.

Flooding and the annual sacrifice of livestock are regular occurrences in Dhaka, meaning most residents have not expressed much shock at these pictures.

Why are animals slaughtered?

The annual Eid al-Adha holiday, the second biggest celebration of the year for Muslims after Eid al-Fitr, commemorates when the prophet Ibrahim showed his readiness to sacrifice his son, Ishmael.

Goats and cows are bought from markets by Muslim families and slaughtered, to thank Allah for sparing Ishmael, after an imam has read verses from the Koran.

The meat is shared between family, friends and the poor, but the off-cuts and offal are discarded.

Has this happened before?

Almost every year there is some amount of blood on the streets as many families slaughter their livestock on the road. It clears away after a few days.

But the heavy rain this year has exaggerated this, making it look as if the streets are awash with blood.

The photos have been shared on social media with some criticising the practice of animal slaughter.

Others defended the custom saying it is a religious duty that also provides livelihood for farmers who sell the livestock and the meat is shared with the poor.

What do the authorities say?

Many residents are criticising city officials for the faulty drainage system, which has long been a problem in some areas.

For their part, city officials have said that there were designated areas where animals should have been slaughtered.

However, some Dhaka residents said there was not enough campaigning to make them aware of these spots. Others said they could not get there because of the rain.

A waste management official with Dhaka South City Corporation told the Dhaka Tribune they were working to resolve the flooding issue.