London's Gatwick Airport, the second busiest airport in Britain, has reopened a terminal after a security alert earlier in the day forced its evacuation and caused travel disruption for thousands of people.

Police sent a bomb disposal team to deal with a suspected prohibited item that they said had been found in luggage at the airport's south terminal, 48 kilometres (30 miles) south of London.

"The earlier security alert has now been resolved and cleared by police," Gatwick said in a statement. 

"The South Terminal is reopening to staff and will be open to passengers shortly."

The incident disrupted weekend travel plans for thousands of passengers, with more than 600 flights due to land or take off on Friday, amounting to more than 121,000 passenger seats, according to data from aviation analytics firm Cirium. 

The airport said some flights were cancelled or delayed and passengers should contact their airline for updates. 

The terminal was closed for more than six hours and the train station serving the airport was also closed as a precaution. Thousands of passengers were seen outside the airport and the surrounding area in videos posted online. Emergency foil blankets were distributed to passengers waiting in the cold, social media pictures showed.

In a separate incident earlier on Friday, London police carried out a controlled explosion near the US embassy in south London after discovering a suspect package. Police later said they believed it was a hoax.