Massive US tariffs have come into force as the condemnation of the Trump administration's move intensifies.
Criticism of the import tax on steel and aluminum from the EU, Canada, and Mexico was joined by top Republicans.
Leaders from affected nations reacted furiously, setting out tit-for-tat tariffs on the US, ranging from steel to sleeping bags and ballpoint pens.
French President Emmanuel Macron told Mr Trump by phone that the US move was "illegal".
Mr Macron told him the EU would respond in a "firm and proportionate manner", the Elysee Palace says.
The French president normally enjoys a good relationship with his US counterpart.
Mr Trump has justified the tariffs, which came into force on Friday, by arguing that US steel and aluminum producers are vital to national security and threatened by a global supply glut.
Bernd Lange, chair of the European Parliament's international trade committee, said the US tariffs were illegal. He hoped a trade war could be avoided but warned that Mr Trump's action demonstrated the president was "not willing to stick to the rules".
Justin Trudeau, the Canadian Prime Minister, rejected the claim that his country posed a national security threat to the US.
What are others saying?
UK International Trade Secretary Liam Fox said the 25% levy on steel was "patently absurd", adding: "It would be a great pity if we ended up in a tit-for-tat trade dispute with our closest allies."
Barry Gardiner, his Labour shadow, told the BBC's Today programme the US measures were "based on a lie", adding the UK should not be "bullied by the president... we believe in a rules-based system and Trump doesn't".
Gareth Stace, head of trade body UK Steel, said the tariffs were "no way to treat your friend" and called on the government to safeguard the industry's 31,000 jobs.
Germany's economy minister, Peter Altmaier, hoped a decisive EU response would make Mr Trump reconsider his decision, while Mexican Foreign Minister Luis Videgaray said his country would also impose duties.
Opposition to the tariffs was also voiced by prominent Republicans. House Speaker Paul Ryan, the most influential Republican in Congress, said the move "targets America's allies when we should be working with them to address the unfair trading practices of countries like China".
What do the US tariffs mean?
Mr Trump first announced plans for the tariffs in March but granted some exemptions while countries negotiated.
On Thursday, US Commerce Secretary Wilbur Ross said talks with the EU, Canada, and Mexico had not made enough progress to warrant a further reprieve, meaning tariffs of 25% on steel and 10% on aluminum have now come into effect.
They apply to items such as plated steel, slabs, coil, rolls of aluminum, and tubes - raw materials which are used extensively in US manufacturing, construction, and the oil industry.
Mr Ross said the president had the authority to lift the tariffs or alter them at any time, leaving room for "flexibility".
"We continue to be quite willing and indeed eager to have discussions with all those parties," he said.