People living in Avellaneda, a town six miles (10 kilometres) from the center of the Argentine capital, described being woken by a stench emanating from the river.

"The smell woke us up. In the daytime, when we looked at this side of the river, it was completely red, all stained," Maria Ducomls told AFP.

"It looked like a river covered in blood, it's horrible," the 52-year-old said.

The Ministry of the Environment for the province of Buenos Aires said in a statement that water samples had been taken to determine what substance had caused the discoloration, citing the possibility of an "organic dye".

The color of the stream, which flows through an area of textile and hide processing factories, had faded by late afternoon, according to an AFP journalist.

"It's terrible, you don't have to be an inspector to see how much pollution the poor Sarandi River suffers from," said Ducomls, who has lived in the area for more than 30 years.

"We have seen the river in other colors -- we've seen it bluish, greenish, pink, purplish, with grease on top that looks like oil," she said, blaming nearby factories for dumping waste into the river.