The United States Postal Service announced Tuesday that it had temporarily stopped accepting packages from China and Hong Kong, hours after an order by President Trump took effect that ended duty-free handling of many of these parcels.

The United States imports about three million parcels a day with little customs inspection and no duties collected — with most of them coming from China. An executive order that Mr. Trump signed on Saturday required that, starting Tuesday morning, each parcel must include detailed information on the contents and the tariff code that applies, as well as payment of those tariffs.

The provision on low-value parcels, known as the de minimis rule, was included in a broader order by Mr. Trump that imposed an extra 10 percent tariff on all imports from China.

But low-value parcels from China, which previously were tariff-free, now face not only the 10 percent tariff but also the many complex tariffs on every category of goods that these shipments previously skirted entirely.

A spokeswoman for the U.S. Postal Service did not respond to a question about whether stoppages were related to the change in trade rules, but only said that service would be temporarily suspended on inbound parcels from China and Hong Kong until further notice.

Customs and Border Protection did not immediately respond to a request for comment. The White House did not immediately provide comment.

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