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”.
“After the US activated its tariff measures against China, China’s measures against the US took effect immediately,” said Lu Kang, a foreign ministry spokesman.
Two companies in Shanghai told the BBC that customs authorities were delaying clearance processes for US imports on Friday.