The Cuban government today put the finishing touches on a mass mobilization that is expected to produce Saturday one of the largest anti-American demonstration here since relations between the two countries were severed in 1961.

Mass rallies, are to begin at 9 a.m. at carefully planned sites in provincial capitals and later cities, across the island.

The government hopes that 5 million Cubans -- half the diminishing population -- will participate and that at least 1 million will gather in front of the U.S. Interest Section on Havana's broad waterfront boulevard.

Several blocks around the U.S. mission have been cordoned off by police since a group of government supporters clashed with a crowd of visa seekers lined up there on May 2. Nearly 100 Cubans fled inside the building and remain there seeking asylum in the United States.

It appears that Saturday the protesters not only will be allowed, but also will be encouraged to surround the building outside a reduced ring of police. Portable toilets have been set up for the event, along with a press viewing area that, to the discomfort of some of the large American press corps allowed to come, is located pratically in the middle of a no man's land between the building and the demonstration site.

In the foreign press room at the Riviera Hotel, the government of President Fidel Castro has provided the somewhat captive journalistic audience with a series of video tapes of films of the original U.S. evacuation of the American Embassy here when relations were broken, recent demonstrations against 50,000 Cubans who have left by boat during the past three weeks, and a blow-by-blow film of the clash at the Interests Section -- which Cuba blames on anti-Castro provocation.

Attendance at the demonstrations has been emphasized in seemingly endless radio and television announcements, on billboards throughout the capital and for the past several weeks at meetings of work, political and neighborhood organizations in which a majority of Cubans participate. the entire front page of today's issue of Granma, the Communist Party paper, was devoted to an editorial letter to citizens urging participation in the "March of the Cuban Combatants" that will "make the foundations of the Yankee interests section tremble." The letter was topped with a headline of the size reserved elsewhere for declaratoins of war, calling on "all of Cuba" to attend.

Below the letter was a page-wide cartoon of marchers carrying placards spelling out three contentious issues the government has made the ostensible centerpieces of the protests: Cuban demands for the United States to lift the 20-year-old trade embargo, to abandon the naval base at Guantanamo and to cease surveillance flights over Cuba.

None of these issues is new, and the degree to which Castro's government places them in the public forum has varied over the years.

But the current level of anti-U.S. sentiment, whipped to a fever pitch by the official propaganda campaign, is reminiscent of Casto's early years in power.

A group of American journalists walking through Havana's international airport terminal after arrival today were subjected to a loud and ostensibly spontaneous barrabe of anti-U.S. jeers and chants that eventually seemed to include every Cuban in the building.

Whether or not the direct cause of the vehement protests is the refugee issue, and allowing for the manipulation and fanning by the government, the anger appears to be genuine.

At the same time, the government has sharply altered the tone and increased the extent of its attacks against the Carter administration. For the first time, the daily political cartoons in Granma, which traditionally depict the United States as an unpleasant-looking Uncle Sam, now features an unflattering likeness of President Carter.

One of several in today's issue showed a big-toothed, wrinkled Carter, an upside down coffee mug perched incongruously atop his head, sitting on the edge of Florida with a ragged banner in his hand.