Stop the Sewage will meet near the Coffee Cup at Eastney on Saturday afternoon following a reported incident last weekend.
Members of the public last Friday reported sewage off Hayling Island, Eastney and Southsea causing the Environment Agency to warn against bathing in the water.
Portsmouth South MP Stephen Morgan said it was 'peculiar' that Southern Water said no evidence of pollution was found, despite a group clearing up 135kg of waste.
Under-fire Southern Water is fresh from a £90m fine earlier this year for historic releases of untreated sewage – with activists accusing the company of further dumps.
Mr Morgan, who has written to the EA about the incident, said: ‘Reports from the weekend are deeply concerning and the quality of our bathing water is a significant concern for many of my constituents.
‘There seems to be a clear difference between what Southern Water is saying and the water local people are witnessing.
‘The government and its regulator need to provide assurances to the people of Portsmouth that our local environment is being properly protected and that industry is living up to its responsibilities.’
Southern said its teams sprang into action 'immediately' to investigate its pumping stations and other assets on Friday night.
All were confirmed, by Saturday, as working normally and there were no storm releases from Budds Farm, a spokesman said.
He added: ‘Our people have continued to visit beaches, shores, outfalls and sites since the reports – sometimes in the company of EA officials. No evidence of pollution has been found.
‘This week we sent crews in boats out to take samples. Some areas of foam were noted which appears to be algae and tests are being carried out. No sewage odour was detected although there is slight odour in some marshland areas possibly due to rotting seaweed or other vegetation.’
The company is spending £17.5m on sewer main renewal and upgrades at a pumping station on Hayling.
It comes as evidence of medical waste was spotted. A 50-year-old man, who asked not to be named, was walking with his eight-year-old daughter on Sunday at Hayling when he spotted syringes.
He said: ‘This waste is clearly freshly dumped in the sea as the syringe still contains what looks like blood and the needles were still sharp.
‘The items were wet and found at the strand-line, mixed in with the washed up seaweed. Very dangerous to children looking for shells.
‘Also potentially dangerous to dogs. We also encountered lots of plastic tampon applicators and cotton buds.’
Portsmouth North MP Penny Mordaunt has taken up the issue with environment minister Rebecca Pow.
As previously reported, councils in the area have been meeting to resolve the problem of storm releases into the Solent.