King Charles III has been crowned at Westminster Abbey in a ceremony built on ancient traditions at a time when the British monarchy faces an uncertain future.

Trumpets sounded inside the medieval abbey and the congregation shouted “God save King Charles” as the ceremony began on Saturday in front of more than 2,000 guests, including world leaders, aristocrats and celebrities.

Outside, thousands of troops, tens of thousands of spectators, and a smattering of protesters converged along a route the king travelled from Buckingham Palace in a gilt-trimmed, horse-drawn carriage.

It was the final leg of a seven-decade journey for Charles from heir to monarch.

To the royal family and government, the occasion — code-named Operation Golden Orb — was a display of heritage, tradition and spectacle unmatched around the world.

The rite was expected to be watched by millions, and many greeted the day with apathy.

Some even met it with disdain. Republican protesters gathered outside to holler “Not my king” of an institution they say stands for privilege and inequality in a country of deepening poverty and fraying social ties. A handful were arrested.