U.S. customs agents began collecting President Donald Trump's unilateral 10% tariff on all imports from many countries on Saturday, with higher levies on goods from 57 larger trading partners due to start next week.

The initial 10% "baseline" tariff took effect at U.S. seaports, airports and customs warehouses at 12:01 a.m., ushering in Trump's full rejection of the post-World War II system of mutually agreed tariff rates.

"This is the single biggest trade action of our lifetime," said Kelly Ann Shaw, a trade lawyer at Hogan Lovells and former White House trade adviser during Trump's first term.