British police shut down London Bridge late June 3 after a van drove into pedestrians in the popular tourist area.
Police were reporting "a number" of injuries, Sky News said a couple of hours later, with the BBC reporting multiple fatalities.
Televised news reports showed police in riot gear clearing the area around London Bridge. They were also evacuating local bars and restaurants.
The Metropolitan Police Twitter feed said officers were also responding to reports of stabbings in Borough Market, next to London Bridge. Shots were fired in that incident, the police said.
Prime Minister Theresa May said the situation was being treated as a "potential act of terrorism," the BBC reported. By 12.30 a.m. local time, the police said the incidents at London Bridge and Borough Hall were being treated as terrorism.
In Manchester, U.K., on May 22 a suicide bomber attacked as people left a concert by pop singer Ariana Grande. There were 23 fatalities, including the bomber, and scores of injuries. A number of those were teenagers and children.
After the Manchester attack, the U.K. raised its threat level to "critical," the highest level available, though that was stepped down to severe a few days later.
Grande was scheduled to return to Manchester on June 4 for a benefit concert.
The incidents come a few days before British voters go to the polls in a general election on June 8. Jeremy Corbyn's Labour Party had reportedly narrowed what had been a commanding lead held by the Conservatives.
In March, a man driving a car fatally wounded four pedestrians near the U.K. Parliament before stabbing a police officer to death and being shot dead himself. In December 2016, a truck was driven into a Christmas market in Berlin, killing 12.
In the worst such incident, in July 2016, a cargo truck mowed down crowds celebrating Bastille Day in Nice, France, killing 86 people.