Twenty-six people were treated for breathing difficulties with two individuals taken to hospital following an alleged CS gas incident at London City Airport.

London Fire Brigade declared the airport safe after completing two sweeps of the area. No elevated readings have been found by police and firefighters who wore protective equipment during the investigation.

The site was evacuated after a fire alarm went off in the terminal building. Passengers and airport staff were left on the tarmac as three fire engines responded to the alarm.

"Three fire engines and a range of specialist appliances have been called to reports of a chemical incident at London City Airport this afternoon," London Ambulance said in a statement.

Ambulance crews and the Hazardous Area Response Team attended to the incident.

We have treated 26 people at the London City Airport incident. We have taken two patients to hospital. — London Ambulance (@Ldn_Ambulance) October 21, 2016

The airport terminal has now reopened and passengers have been advised to check with their airline for the latest information.

Update: Evacuation still underway passengers are advised to check with their airlines for latest flight information https://t.co/AvhFR0TDE6 — London City Airport (@LondonCityAir) October 21, 2016

Police are investigating the incident and believe they found CS gas spray.

"A subsequent search of the airport led to the discovery of what is believed to be a CS gas spray,” Met Police said.

"Whilst the cause of the incident has not yet been confirmed, officers are investigating whether it was the result of an accidental discharge of the spray.”

Police say they are looking into whether it may have been discarded by a passenger before check-in.

Terminal has been evacuated due to fire alarm, we are looking into the cause- updates to follow. — London City Airport (@LondonCityAir) October 21, 2016

London City Airport tweeted police are investigating “whether it was the result of an accidental discharge of the spray.”

Images shared on social media Friday show crowds gathered outside the airport after the alarm rang at the busy airport in central London.

An other one #firealarm A photo posted by Icicle (@icicle_music) on Oct 21, 2016 at 8:31am PDT

Firealarm at @londoncityair ! Bit surreal! A photo posted by Icicle (@icicle_music) on Oct 21, 2016 at 8:12am PDT

City airport closed... great start to ADE😡 A photo posted by Russ Yallop (@russyallop) on Oct 21, 2016 at 8:58am PDT

Airport got evacuated, can't get on my flight back to Frankfurt. Apparently someone is spreading poisoning gas here, there's ambulances everywhere. #forever #delay A photo posted by @nty722 on Oct 21, 2016 at 8:56am PDT

An other one #firealarm A photo posted by Icicle (@icicle_music) on Oct 21, 2016 at 8:31am PDT

On Thursday, a 19 year old was arrested at a nearby tube station in North Greenwich after police carried out a controlled explosion.