Around 900 litres of the poisonous substance flowed into Årstaviken, the body of water south of Södermalm, after an accident on a nearby industrial estate.
Technicians from Stockholm Vatten, which supplies the city’s water, joined safety personnel at the scene but assured the public that there was no risk that the chemical would get into the drinking water.
Hydrogen peroxide is used in smaller concentrations in hair bleach, among other products.
“This sort of substance is broken down by water,” said Ulrika Österberg, an environmental assistant at Stockholm Vatten.
“The drains that this relates to run out into Årstaviken, a long way from the places from where we take our drinking water.”
The company responsible for the container alerted the emergency services just after 9am on Tuesday and police, ambulance and fire services worked at the scene all morning. One man was taken to hospital and employees at the company were told to stay indoors.
“I think the worst that will happen is that a few fish will be affected,” said Ulrika Österberg.