US and China fire first shots in $34bn trade war
The 25% levy came into effect at midnight Washington time.
China has retaliated by imposing a similar 25% tariff on 545 US products, also worth a total of $34bn.
Beijing accused the US of starting the "largest trade war in economic history".
"The Chinese side, having vowed not to fire the first shot, is forced to stage counter-attacks to protect the... interests of its people," the Chinese Commerce Ministry said.
Earlier this year, President Donald Trump ordered the US to impose tariffs on $50bn worth of Chinese goods and the first round of these kicked in on Friday.