Tonnes of dead fish have floated to the surface since July 15, leaving a stench hanging over the city centre. Even eels, the Tiber's hardiest denizens, have leapt onto the banks to escape the water.

"It's an apocalypse, I've never seen anything like it," said Carmen Di Penta, director of Marevivo, an environmental group that has monitored the Tiber for 12 years.

"We think some 60 to 70 percent of the fish may have died. It's going to take years to get it back to the way it was," she told Reuters.

Environmentalists, civic authorities and lawyers are divided over the cause of the problem, but agree it began two weeks ago when torrential rains lashed central Italy.

Magistrates in Rome, conducting an official investigation, say the rains caused a surge of water that flushed sewage, nitrates and other pollutants down the Tiber.

The sewage fed the algae, which sapped oxygen from the water, leaving the fish unable to breathe.

"There's no evidence of any 'poisons', pesticides or other toxic substances," the magistrates said in a statement. "Nor did we find any traces of pathogens in the dead fish."

They said levels of pollution have already fallen.

But city officials are not convinced. They suspect the problem lies in the Aniene, a badly polluted tributary of the Tiber, northeast of Rome.

"When we saw the number of dead fish, we thought it had to be caused by poison or some industrial accident," said a spokeswoman for the environmental office of Rome's city hall.

"The Aniene is filthy, not just with industrial pollutants but with people illegally dumping their waste."

Green activists say the Tiber surges almost every year but never with such terrible consequences.

The debate looks set to continue. City officials say the provincial government, which is responsible for the Aniene, should be brought to task.

Whatever the outcome, Marevivo says the pollution has contributed to the decline of one of history's great rivers.