Four people were killed and at least 20 injured after a car ploughed into pedestrians and an attacker stabbed a policeman, in what police called a terrorist incident.
"The thoughts of the PM and the government are with those killed and injured in this appalling incident, and with their families," her office said in a statement.
"The PM is being kept updated and will shortly chair COBR," the statement said, referring to a meeting of the government's emergency security committee to be held later on Wednesday.
“People's security top priority”
The British people's security is the government's top priority, interior minister Amber Rudd said after four people were killed and 20 injured in London during an attack close to parliament on Wednesday.
"The government's top priority is the security of its people," she said in a clip broadcast on Sky News.
"The British people will be united in working together to defeat those who would harm our shared values: values of democracy, tolerance and the rule of law."
Trump offers US cooperation, support
President Donald Trump on Wednesday offered British Prime Minister Theresa May the full cooperation and support of the United States in responding to an attack close to the British Parliament, the White House said in a statement.
Merkel expresses shock
German Chancellor Angela Merkel expressed shock at the attack in which four people were killed and at least 20 injured in London on Wednesday, and she said her thoughts were with the British people and everyone in the capital.
In the attack close to the British parliament a car ploughed into pedestrians and an attacker stabbed a policeman, in what police called a terrorist incident.
Merkel stressed that Germany was on the same side as Britain in terms of fighting terrorism. In December, a failed Tunisian asylum seeker pledging allegiance to Islamic State killed 12 people in Berlin by driving a truck into a crowded festive market.
"Although the background to these incidents still needs to be exactly clarified, I confirm on behalf of Germany and its citizens that we stand firmly and resolutely by Britain's side when it comes to fighting any kind of terrorism," she said in an emailed statement.