Sadiq Khan was asked about the treatment of Pastor Olu during Mayor's Question Time at the London Assembly on Thursday.
He said: "I expect the police to take this matter seriously and I can write to members once an update on this investigation has been received."
The mayor was questioned on the matter by London Assembly member and UKIP politician, David Kurten.
Mr Kurten, a Christian, said he was "troubled" to learn of the arrest of the preacher, whose full name is Oluwole Ilesanmi.
In a written submission, he said: "I am disappointed that any members of the Metropolitan Police would think it appropriate to behave in this manner.
WATCH! I ask @sadiqkhan how he is going to celebrate the glorious occasion of the United Kingdom leaving the European Union on 29th March 2019. https://t.co/8u6iWpwWll
— David Kurten (@davidkurten) March 21, 2019
"What is your view of the actions taken by the police on this particular occasion?"
Officers said the brief detention of Mr Ilesanmi outside Southgate Underground station on Saturday 23rd February was made to "prevent a breach of the peace".
The 64 year old has maintained that he did nothing wrong.
A Christian preacher this afternoon was arrested at Southgate station London N14 for preaching about Jesus . @CConcern the police told him it's because public had said he was being islamophobic . We have the full video and wondered when preaching was now illegal ? @Campaign4T pic.twitter.com/ZxC1DhlZCf
— Eye On Antisemitism (@AntisemitismEye) 23 February 2019
Footage of the incident, which has been seen more than 2.4 million times, appears to show Mr Ilesanmi being accused of "being racist" but it's unclear what he said while preaching which caused the police to intervene.
Earlier this month, the Met said its local professional standards unit was investigating what happened.
Street Preacher Olu about to head into City Hall to hear what @SadiqKhan has to say about his arrest.
— Christian Concern (@CConcern) March 21, 2019
You can follow here https://t.co/5qxrBDbXx4 pic.twitter.com/xwHf9MUyWl
Sadiq Khan it would be "inappropriate" for him to comment on the specifics of the case, given a complaint is being investigated.
But he said: "It's always been clear that freedom of speech is critical and must be protected but also the police must be able to investigate when allegations are made of a criminal offence.
We are here in City Hall waiting for questions to @SadiqKhan on street preacher Olu's arrest. #FreedomOfSpeech pic.twitter.com/vUodcbnU61
— Christian Concern (@CConcern) March 21, 2019
"Police officers have to strike a difficult balance on this issue on a regular basis and in complex circumstances, such as policing protests and public order incidents."
He added: "...our police officers are always expected to uphold the highest standards."
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