The Metropolitan Police's Twitter account was targeted by hackers who posted multiple bizarre and unauthorised messages.
Late on Friday night, a number of posts were sent from the force's Twitter account, which has more than 1.22 million followers, calling for the release of drill rap artist Digga D.
One post, which has now been deleted, read: "We aim to make London the safest global city; Be the best crime-fighters, by any measure; Earn the trust and confidence of every community; Take pride in the quality of our service; So people love, respect and are proud of London's Met.'
Another said: "FREE DIGGA D ON FOENEM GANG."
Scotland Yard said in a statement there had been no "hack" of its IT infrastructure and the security issue had only affected its MyNewsDesk account, which it uses to issue news releases and information to the media.
"Last night, Friday 19 July, unauthorised messages appeared on the news section of our website as well as on the @metpoliceuk Twitter feed and in emails sent to subscribers.
"While we are still working to establish exactly what happened, we have begun making changes to our access arrangements to MyNewsDesk," it said.
Some tweets shared fake press releases, containing anti-police sentiments and calling for Digga D's release.
"We apologise to our subscribers and followers for the messages they have received," the force added.
"At this stage, we are confident the only security issue relates to access to our MyNewsDesk account.
"We are assessing to establish what criminal offences have been committed."
Earlier, a Scotland Yard superintendent had said the Met's official account has "been subject to unauthorised access".
Superintendent Roy Smith tweeted: "Our media team are working hard to delete the messages and ensure the security of the account. Please ignore any Tweets until we verify that it is back under official control."