King Charles and The President of the Portuguese Republic celebrated the diplomatic ties between the two countries at the palace, said a statement.
The palace said the pair began Portugal-UK 650 celebrations at a combined Guard of Honour at the Buckingham Palace.
The statement said,"Today, commemorations mark 650 years of the Anglo-Portuguese Alliance, the oldest diplomatic alliance in the world still in force."
The King and President Marcelo Rebelo de Sousa later attended a Service of Thanksgiving to mark the anniversary at The Queen’s Chapel in St. James’ Palace.
"To date, Portugal-UK 650 has organised more than 300 activities and projects in both countries, in collaboration with 210 institutions," the statement said.
It said The Queen’s Chapel was the Chapel of the Portuguese-born Queen Catherine of Braganza, wife of King Charles II.
Her coat of arms, combining the arms of Portugal and the United Kingdom, features above the altar and in the Gallery.
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