Police are investigating whether items found near the Israeli embassy in central London are connected to a video posted on social media claiming the building was going to be targeted by drones carrying dangerous substances.
The Metropolitan Police said the embassy had not been attacked and officers were trying to verify the source and authenticity of the footage.
A large number of police officers were deployed to the park and could be seen from behind a cordon.
The embassy confirmed the site was not attacked and staff were safe, while the Met said it did "not believe there to be any increased public safety risk at this stage".
A cordon was installed around the site on Friday morning and the park was expected to remain closed throughout the day.
A Met Police chemical, biological, radiological and nuclear team was seen near the park's bandstand and police divers were spotted near the Round Pond ornamental lake.
London Fire Brigade's fire investigation dogs were also deployed to the scene, as well as the London Ambulance Service's hazardous area response team.
Met Police deputy commissioner Matt Jukes said: "Counter-terrorism policing London are aware of a video shared online overnight in which a group claim to have targeted the nearby Embassy of Israel... we're carrying out urgent inquiries to determine the authenticity of the video and to identify any potential link between it and the items discarded in Kensington Gardens."
The force urged people to avoid the area while investigations were carried out.
Jeffrey, a local resident, told BBC News he was concerned "because we don't know what it is".
"This is a place where people come to relax and have a nice time... having the park closed has an effect on people and we don't know when it'll reopen," he said.
A video posted to social media, which appears to have been posted by Iran-linked Islamist group Harakat Ashab al-Yamin al-Islamia, showed a target over an image of the Israeli embassy before people could be seen in the footage dressed in hazmat suits flying drones.
The group has also posted videos on social media about several other incidents in London, including an arson attack on four Jewish ambulances in Golders Green, an attempted arson attack on a synagogue in Finchley and an attempted arson attack on a Persian media company in Park Royal.
However, police are not currently linking this incident to any other attacks, the force says.
Shadow home secretary Chris Philp called on the government to set out what steps are being taken to protect the embassy.
"The relentless intimidation of the Jewish community and attacks on Jewish institutions have absolutely no place in this country," he said.
A spokesperson for the embassy said: "A suspected security incident is being investigated in the park adjacent to the Embassy of Israel in London.
"We wish to clarify that all embassy staff are safe and that the embassy was not attacked.
"As always we remain in close and continuous contact with the local authorities."
Kensington Palace, where the Prince and Princess of Wales used to have their main home, is located on the western edge of Kensington Gardens.
Staff from the royal couple's household based at the historic site are still at work there.
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