On a state visit that mixed royal pageantry with tough political talks on Ukraine and migration, Macron said Europe must strengthen its economy and defences and reduce its dependence "on both the US and China".
Macron's three-day trip, at the invitation of King Charles III, is the first state visit to the UK by a European Union head of state since Brexit, and a symbol of the UK government's desire to reset relations with the bloc after Britain acrimoniously left the EU in 2020.
Macron addressed members of both houses of Britain's parliament packed into the building's ornate Royal Gallery. He said the two countries represent "a world order based on law, justice and respect for territorial integrity, an order that is today being attacked on a daily basis".
"The United Kingdom and France must once again show the world that our alliance can make all the difference," Macron said, adding that "we will save Europe by our example and our solidarity".
He said that even though Britain has left the EU, "the United Kingdom cannot stay on the sidelines. Because defence and security, competitiveness, democracy - the very core of our identity - are connected across Europe as a continent."
The French president and his wife, Brigitte Macron, were treated to the full force of British ceremonial charm, a far cry from the chilly relations of 2022 when then-foreign secretary Liz Truss said that the "jury is out" on whether Macron was a friend or a foe.
The Macrons were greeted at London's RAF Northolt air base by Prince William and his wife Catherine - wearing a dress by French design house Christian Dior - before being met by King Charles and Queen Camilla in Windsor, west of London.
They were driven to the almost 1000-year-old royal residence of Windsor Castle in horse-drawn carriages, through streets bedecked in Union Jacks and French tricolour flags.
The king and queen hosted a banquet for the Macrons at Windsor Castle on Tuesday evening, with 160 guests including politicians, diplomats and celebrities such as Mick Jagger and Elton John.
They were served summer vegetables, chicken with asparagus and iced blackcurrant parfait, along with champagne and a gin-infused cocktail called L'entente, after the "entente cordiale" struck between Britain and France in 1904.
As monarch, King Charles is expected to be above politics, but he spoke about the support Britain and France give Ukraine "in defence of our shared values", noted the challenge of "irregular migration across the English Channel" and said the two countries face "complex threats, emanating from multiple directions".
"As friends and as allies, we face them together," he said.
