AMSTERDAM — A policy barring foreign tourists from buying marijuana in the Netherlands went into effect in parts of the country Tuesday, with attention focused on the southern city of Maastricht, where a cafe was warned over violating the ban and around 200 smokers marched in protest.

Weed is technically illegal in the Netherlands, but it has been sold openly for decades in small amounts in designated cafes known as “coffee shops” under the country’s famed tolerance policy.

Under a government policy change, as of May 1, only holders of a “weed pass” are supposed to be allowed to purchase the drug in three southern provinces. Nonresidents aren’t eligible for the pass, which means tourists are effectively banned.

The policy isn’t supposed to go into effect in Amsterdam, home to around a third of the country’s coffee shops, until next year — and it may never be. The city opposes the idea and the conservative national government collapsed last week, raising questions about whether a new Cabinet will persevere with the policy change after elections are held in September.

Most attention Tuesday was on the city of Maastricht, which borders both Belgium and Germany and which has suffered the effects of a constant flow of traffic from more than a million non-Dutch Europeans driving to the city annually just to purchase as much cannabis as possible and drive back home.

Most shops in Maastricht plan to refuse to use the pass and kept their doors shut Tuesday.