They kicked in just after midnight ET, which is noon in Beijing. Another $16 billion worth of tariffs on Chinese imports are set to hit in the next two weeks.
U.S. officials accuse China of building its industrial dominance by stealing the "crown jewels" of American technological know-how through cyber-theft, forced transfers of intellectual property and state-sponsored corporate acquisitions.
Beijing insists it's the injured party.
"China promised not to fire the first shot, but to defend national core interests and the interests of the people; it has no choice but to strike back as necessary", said the commerce ministry in a statement. It accused the United States of "typical trade bullying".
Chinese ports delay U.S. cargoesSome major Chinese ports delayed clearing goods from the U.S. on Friday, four sources said, potentially disrupting imports worth billions of dollars as the world's top two economies head towards an outright trade war.
Alas, the tariffs going into effect Friday will probably punish us and our allies more than they'll hurt China.
American companies doing business in China are particularly anxious.
The American Soybean Association (ASA) said in a statement that soybean farmers "rely heavily" on exports to China, and said that they lobbied Trump to reconsider the tariffs.More news: Belgium paid price for missed chances in semifinal defeat - Roberto Martinez
The big question is how far the hostilities between Washington and Beijing will go.
This will be affected by how high individual tariffs are, and also if they are expanded to other countries like Canada, Mexico and the European Union.
In the run-up to Friday, there was no sign of renewed negotiations between USA and Chinese officials, business sources in Washington and Beijing said.
The president upped the stakes on Thursday, saying the amount of goods subject to tariffs could rise to more than $500bn. "Okay? So, we have 50 plus 200 plus nearly 300", he told reporters on Thursday.
Chinese state-run media reported that Beijing's measures against Washington formally took effect at 12:01 p.m. on Friday at the same time as the United States activated its tariffs against China.
The American Chamber of Commerce in China appealed to both sides to negotiate.
Some economists question the accuracy of the official figures, yet the US Census Bureau shows America bought $376bn more of Chinese goods than the total value of all US goods sold in the Asian country past year.More news: Trump arrives in Brussels ahead of potentially contentious North Atlantic Treaty Organisation summit
With only $130 billion in U.S. imports to retaliate against, Beijing has said it will take "qualitative" and "quantitative" measures against the United States, triggering fears it could cripple the operations of USA multinationals in China.
As the trade war goes on, or even grows, in the next few months, movement in those two approval numbers will likely reveal a lot about the fate of Trump's presidency.
While some see this as a powerful bargaining chip with Washington, it is a double-edged sword for Beijing as dumping the bonds would cause self-inflicted losses.
Touching on China's massive trade surpluses with the United States, Trump told his audience, "They've been killing us".
President Donald Trump said more than $500 billion in Chinese products could ultimately be penalized.
For his part, Li said that China would further open its economy to foreign investment.
Kennedy say he thinks the Trump administration's enthusiasm for the conflict "will erode as the economic pain and political fallout from a trade war begin to take hold".
The head of the Asia-Pacific Committee of German Business (APA), Hubert Lienhard, said that in the face of a growing protectionist agenda in the U.S., a joint German-Chinese message to the White House "could be one the of things emerging from today's government consultations".More news: George Clooney recovering after scooter accident in Sardinia