The King has made an impassioned plea for religious tolerance and mutual respect, against the background of "international turmoil" in Israel and Gaza.
In a speech at Mansion House, in the City of London, he spoke of the "heartbreaking loss of life".
The King has long supported building bridges between faiths, calling the UK a "community of communities".
But he also spoke of the importance of "our ability to laugh at ourselves".
In particular, he highlighted his own problems with malfunctioning fountain pens.
The King called for "an invigorating dash of self-irony" and hailed the importance of a sense of humour as part of the national character.
This was particularly relevant in his own case, the King said, after the "vicissitudes I have faced with frustratingly failing fountain pens this past year".
Signing a visitors' book in Belfast, he and was overheard saying: "Oh God I hate this... I can't bear this bloody thing."
Addressing the City of London's lord mayor and representatives, the King called for the moderating forces of "civility and tolerance, on which our political life and wider national conversation depend".
He warned against the "rancour and acrimony" of social media, with its angry extremism, and the risk of becoming a "shouting or recriminatory society".
There was a particular call for respect between different faiths and cultures - and since the Hamas attack on Israel, the King has spoken to King Abdullah II of Jordan, Israeli President Isaac Herzog and chief rabbi Ephraim Mirvis.
The King called for a "breathing space" to allow people to "think and speak freely" and for disputes to be "passionate but not pugnacious".
One of his first acts as King had been to invite a range of religious leaders into Buckingham Palace, he said, and he wanted to "rededicate my life to protecting the space for faith".
In his plea for traditions of calm and respect, the King warned against the trashing of public institutions and public service and the threat of "demotivating scapegoating" for those working in them.
Since the "dawn of history" Britain had been "enriched by our welcome of new citizens from the four corners of the globe", he added.
The Mansion House event marked the symbolic arrival of a new monarch in the City of London.
There was a ceremony, dating back to the 14th Century, in which the Pearl Sword is presented to the monarch, who then returns it to the lord mayor.
The sword is in a scabbard covered with 2,600 pearls and the ceremony is a symbolic show of mutual respect between two historical powerbases, the monarchy and the City of London.
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