President Donald Trump should understand the U.K. will always be prepared to challenge his views in a respectful way — regardless of whether his feelings get hurt, London Mayor Sadiq Khan told CNBC Friday.

“The great thing about our city is you can have a fraternal and positive relationship with a country and its citizens but disagree in a mature and adult way with the president,” London Mayor Sadiq Khan told CNBC’s Wilfred Frost on Friday morning.

“One of the great things about having a best friend as we do with America is we can call you out when we think you’re wrong … I think we shouldn’t be afraid of doing so,” he added.

The U.S. president is currently visiting the U.K. as part of a working visit to meet with Prime Minister Theresa May, Queen Elizabeth II and several other business leaders. He is then expected to travel to Scotland for a weekend of golf.

Shortly after arriving in the U.K. on Thursday afternoon, Trump sought to reignite a long-running feud with London’s mayor.

“Take a look at the terrorism that is taking place. Look at what is going on in London. I think (Khan) has done a very bad job on terrorism,” Trump told The Sun newspaper in an interview published late Thursday.

“I used to love London as a city. I haven’t been there in a long time … But when they make you feel unwelcome, why would I stay there?”

The U.S. president has decided not to tour the U.K.’s capital city during his working visit after hundreds of thousands of Britons vowed to take to the streets to protest against him.

The main “Stop Trump” rally is expected to attract around 70,000 people on the streets of London on Friday.

When asked what one word he would use to describe Trump’s time in office, Khan replied: “Interesting.”

He went onto add: “It is not for me to respond to the tweets President Trump sends or some of the interviews he gives where he mentions me and he talks about me in ways which I find surprising.”

Meanwhile, a “Trump Baby” blimp was seen flying beside U.K. Parliament on Friday morning, after organizers raised more than £30,000 ($40,000) to pay for its manufacture and inflation.

Khan has since defended his decision to grant permission for it to be flown as part of a protest against Trump’s visit.

The balloon depicting the U.S. leader as giant baby is tethered to the grounds of Parliament Square Gardens and will be restricted from floating higher than 30 meters (98 feet). It will fly between 9:30 a.m. and 11:30 a.m London time.