The campaigners gathered outside its Yeoman Road headquarters in Worthing from noon to argue that 'time's up' on water privatisation.
The demonstration follows the news last month that Southern Water had been forced to pay £126m for 'spilling wastewater into the environment' and 'deliberately misreporting its performance'.
A Sussex Police spokesman confirmed that members of the police prevention team went along to assess the scene and found 20 people protesting at the building.
The protest ended at 1pm, the spokesman said.
The demonstration was part of a national day of action by We Own It - a national campaign for public ownership of public services. Similar demonstrations will be taking place at other water company headquarters across the country.
We Own It's campaigns officer Ellen Lees said the privatisation of water companies had been a failure.
"We’ve had thirty years of rising bills, shareholders and CEOs pocketing millions, and private companies failing to fix leaks in the system," she said.
“The public are absolutely clear. Eighty-three per cent of us want to see water in public ownership.
"Now we’re speaking out and saying time’s up for private water companies that have ripped us off, polluted our rivers and allowed cash to flow straight from our bills into their shareholders’ bank accounts.
"Now we’re saying it’s time for public ownership."
The day of action marks 30 years since the water industry was privatised by Margaret Thatcher's government and comes after a story widely reported that the CEOs of private water companies have earned £70 million in salary, bonuses and other remuneration in the past six years.
We Own It argues that bringing England’s water companies into public ownership would save £2.3 billion every year.
They also claim that this could save people around £100 a year on bills.
The demonstrations are taking place ahead of the launch of the group's People’s Plan for Water in parliament on Wednesday, July 10.
The People’s Plan is a crowdsourced manifesto for what publicly owned water could look like and what it could achieve.
MPs including Clive Lewis, Deidre Brock and Luke Pollard are all confirmed to attend the launch.