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 to be paid by U.S. importers took effect at U.S. seaports, airports and customs warehouses at 12:01 a.m. ET (0401 GMT), ushering in Trump's full rejection of the post-World War Two 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.

Shaw told a Brookings Institution event on Thursday that she expected the tariffs to evolve over time as countries seek to negotiate lower rates. "But this is huge. This is a pretty seismic and significant shift in the way that we trade with every country on earth," she added.

Trump's Wednesday tariff announcement shook global stock markets, wiping out $5 trillion in stock market value for S&P 500 (.SPX), opens new tab companies by Friday's close, a record two-day decline. Prices for oil and commodities plunged, while investors fled to the safety of government bonds.

Among the countries first hit with the 10% tariff are Australia, Britain, Colombia, Argentina, Egypt and Saudi Arabia. A U.S. Customs and Border Protection bulletin to shippers indicates no grace period for cargoes on the water at midnight on Saturday.

But a U.S. Customs and Border Protection bulletin did provide a 51-day grace period, opens new tab for cargoes loaded onto vessels or planes and in transit to the U.S. before 12:01 a.m. ET Saturday. 

These cargoes need arrive by 12:01 a.m. ET on May 27 to avoid the 10% duty.

At the same hour on Wednesday, Trump's higher "reciprocal" tariff rates of 11% to 50% are due to take effect. European Union imports will be hit with a 20% tariff, while Chinese goods will be hit with a 34% tariff, bringing Trump's total new levies on China to 54%.

Beijing on Saturday said "the market has spoken" in rejecting Trump's tariffs after it hit Washington with a slew of countermeasures, including extra levies of 34% on all U.S. goods and export curbs on some rare earth minerals.