LONDON: London City airport was declared safe on Friday after hundreds of passengers had to be evacuated and two were taken to hospital with breathing difficulties in an incident that police suspect was caused by CS gas.

The airport was briefly closed as police and firefighters in protective equipment swept the terminal building of the airport with chemical detectors after several people were taken ill, some of them coughing violently.

"Two complete sweeps of the airport building were carried out jointly by firefighters and police officers ..," the fire brigade said. "No elevated readings were found and the building was ventilated, searched and declared safe."

Later, police said the incident may have been caused by CS gas, used for self-defense and crowd control.

"A search of the airport led to the discovery of what is believed to be a CS gas or spray. Whilst the cause of the incident has not yet been confirmed, officers are investigating if it was the result of an accidental discharge of this canister," police said in a statement.

"At this early stage, officers are investigating whether it may have been discarded by a passenger prior to check-in. There have been no arrest and inquiries continue," the statement said.

The airport later said it had reopened but that flights would be staggered and some disruption was expected.

Ambulance teams trained in dealing with hazardous substances raced to the airport at around 1500 GMT.

Medics said they treated 27 people for breathing difficulties at the airport itself and took two to hospital.

Among those caught up in the incident was boxer David Haye who Tweeted: "CityAirport got evacuated when everyone started coughing uncontrollably!"

The fire brigade said about 500 people had been evacuated from the terminal of the airport east of London's Canary Wharf financial district, which mostly serves short-haul, European destinations.

Photos posted on social media showed many passengers milling around on the tarmac, some sheltering from the rain under the wings of parked planes.

The incident came shortly after police said on Friday they had arrested a teenager under terrorism laws following the discovery of a "suspicious item" on a London underground train near Canary Wharf.

Officers discharged a stun gun during the arrest of the 19-year-old in north London who was detained on suspicion of preparing terrorism acts.

The suspect item, which is still being forensically examined, was found by staff on Thursday morning on a train at North Greenwich station in east London near Canary Wharf and close to the O2 music venue.

The station was evacuated and bomb squad officers carried out a controlled explosion to make the item safe. The investigation into the incident is being led by London's Counter Terrorism Command.