Facebook's Safety Check feature was activated Saturday after a van drove into a crowd on London Bridge.
Local law enforcement confirmed that more than one person had been killed, according to the BBC, which also said there had been reports of multiple casualties. London's Metropolitan Police tweeted that the bridge attack and an attack at Borough Market had been declared terrorist incidents.
The United States Embassy in London posted a tweet telling US citizens in Britain: "If you're safe, please notify friends & loved ones. If you use social media, update statuses." It included a link to Safety Check.
The feature sends a message to Facebook users in areas of immediate danger, allowing people to notify friends and family that they're alive and well. It has been used for natural disasters like hurricanes, as well as mass shootings and terrorist attacks.
The Facebook tool has been activated more than 300 times in the past three years, but the social network has also been criticized for failing to activate Safety Check in some parts of the world. In response, Facebook no longer manually activates the tool, relying instead on user activity to automatically trigger alerts, tracking keywords indicating danger, like "earthquake," "shooting" or "explosion" in concentrated areas.
CNET's Steven Musil contributed to this report.