LONDON (AP) -- Holding signs with slogans including "stop the plop" and "species not feces," thousands of people marched in London on Sunday to demand a cleanup of Britain's sewage-clogged waterways.

Organizers estimated that 15,000 people took part in the march along the River Thames to Parliament, many wearing blue and waving blue flags to form a "human river." Police did not give an estimate of the crowd.

The March for Clean Water was organized by groups ranging from Greenpeace and Friends of the Earth to British Rowing.

Water pollution has become an increasingly high-profile issue in Britain, focusing attention on climate change and the legacy of privatization of Britain's utilities several decades ago.

Private companies that provide both water and sewage have failed to update their often-Victorian infrastructure as populations have grown and demand swelled. Leaks are common, and during heavy rainfall companies discharge raw sewage into rivers, lakes and the sea. The number of discharges increased by more than 50% last year to a record 464,000 spills, according to the Environment Agency.

Pollution from farm runoff also taints Britain's waterways, and climate change is making the situation worse by bringing more intense rain.

Rower Imogen Grant, a gold medalist at the 2024 Olympic Games, said it was commonplace for rowers to get sick after training on the Thames.

"I spend hours and hours on the water training every day in the rowing boats during my sessions, and seeing nappies floating along, seeing plastic bags, seeing scum along the pontoons that we're training on ... It's just not good enough and something needs to change," she said.

Water companies say the industry regulator won't allow them to raise water bills enough to finance improvements.

The Labour Party government, elected in July, has introduced a bill to strengthen regulation and impose tougher penalties on water companies responsible for pollution. But the demonstrators said more needs to be done.

Nature broadcaster Chris Packham, who attended the march, said Britain's rivers are "among the worst in Europe."

"But I'm hopeful, firstly because we know what we need to do, and we have the technologies to fix it. What we need to do is to convince our new government to act more rapidly," he said.