One person was killed and eight others injured when a van was driven into people near the north London mosque shortly after they had just finished worshiping after breaking the Ramadan fast.
A 48-year-old man was arrested and the attack in the Seven Sisters Road is being investigated by counter-terrorism police.
Describing it as "terrible incident", the prime minister said:"All my thoughts are with those who have been injured, their loved ones and the emergency services on the scene."
She later said police were treating the van incident "as a potential terrorist attack", adding: "I will chair an emergency meeting later this morning. All my thoughts are with the victims, their families and the emergency services on the scene."
Jeremy Corbyn, the Labour leader and local Islington North MP, expressed his shock at what had happened.
"I'm totally shocked at the incident at Finsbury Park tonight," he said.
"I've been in touch with the mosques, police and Islington council regarding the incident. My thoughts are with those and the community affected by this awful event."
Emergency services are on the scene and investigating a major incident at Finsbury Park.
Sadiq Khan, the London mayor, said, "Emergency services are on the scene and investigating a major incident at Finsbury Park."
Source: The Telegraph