Demonstrators wore poo hats, fish heads and toilet roll costumes for the stunt at Shirehall, where a Severn Trent Water boss was speaking at a Shrewsbury Town Council meeting.
Around 20 protestors were in attendance, banging drums and holding placards with slogans including "cut the c***", "don't treat us like s***" and "make the polluters pay".
One campaigner, Pete Griffith, said: "We're raising the issue of Severn Trent not doing enough to stop raw sewage getting into the River Severn in Shrewsbury. They are infecting the environment and affecting the whole town.
"It's a lovely town, but it's going to be known as 'S***** Shrewsbury."
Fellow campaigner Claire Kirby, wore a fish head to the protest. She said: "We've got fish heads and sewage (costumes) because fish and sewage don't mix.
"Unfortunately, more and more sewage is going into our river as time passes, causing problems for both human health and wildlife.
"Shrewsbury Town Council are having a meeting and they've got the managing director of Severn Trent along to talk about the sewage situation in our rivers.
"We need them to do something before it's too late."
Severn Trent managing director James Jesic delivered a presentation at the meeting, in which he spoke about improvements and investment the company is making, saying that "we can all play our part" in cleaning up the river.
However, Claire told him she had been calling for something to be done for years in the Coton Hill area, and that a lack of action had forced residents to launch the campaign group.
Coton Hill councillor Nat Green also described the situation "a disgrace", and told Mr Jesic: "We need to know when you are going to put an end to this. It is not good enough."
Belle Vue councillor Kate Halliday, whose ward is plagued by flooding issues, told Mr Jesic she felt he presented Severn Trent in a "glossier" light, pointing out that the company was fined £1.5m for leaking sewage in Worcestershire.
She added: "Your company is making a lot of money and the CEO is making a lot of money," and that it seems like the firm is putting profit before river safety."
Mr Jesic told councillors and others in attendance that rivers are improving overall, but "we need to ramp up the pace".
He said that the water sector has cut serious pollution by 90 per cent in the last 30 years, but there are still too few rivers with "good" status regarding pollution.
Severn Trent says its plans to improve rivers involves £12bn investment in improving the overall sewerage system and £566m to alleviate flooding.
The protest group joined forces against the issue of raw sewage being dumped in the River Severn late last year. They have held demonstrations including a march along the river in which they banged drums and carried placards with slogans including 'Stop the Poonami'.
After that protest, Claire said: “As Shrewsbury residents, we’re fed up of seeing our beautiful river, and the footpaths around it, ruined by human excrement and sanitary waste.
"The Severn is the jewel in Shrewsbury’s crown and loads of people, including children, swim, canoe and paddleboard on it. So why are we letting Severn Trent Water treat it like a toilet? Since privatisation in 1991, water company shareholders have made £57 billion in dividends, while failing to fix our Victorian sewer system. We have had enough."