First McDonald's had to agree to buy Mexican potatoes for the french fries.

Then commentators warned that the country faced "a hamburger invasion" that "threatened national values" as hundreds of cars lined up day after day for Big Macs and the other familiar treats.

Now red and black strike banners fly beneath the golden arches, and "Death to the voracious bosses!" is painted on one of the walls, as a restaurant union has shut down Mexico's first McDonald's in a show of labor muscle and nationalist pride.

It has been tough going for McDonald's first foray into the Mexican market. The Illinois-based company operates in 48 countries but waited until Oct. 29 to open in Mexico, whose welcomes to foreign corporations are often frosty.

The McDonald's case represents a skirmish in the ongoing conflict in Mexico over how much to open the economy to foreign, and particularly U.S., investment and economic influence. The union is seeking to protect its position in Mexico's heavily organized restaurant industry, where McDonald's is trying to introduce a "fast food" labor model using part-time, nonunion workers.

In addition, the case illustrates Mexico's ambivalence about adopting U.S. life styles and values, according to union activists, management representatives and economic analysts. "There are obvious ideological implications for starting a strike there. McDonald's is a well-known trademark, and from the beginning it was taken as a symbol of U.S. investment and intervention," a private sector research economist said.

Ironically, McDonald's does not even have a majority interest in the restaurant here. A controlling share of 51 percent is owned by a Mexican entrepreneur in his twenties named Saul Kahan.

But McDonald's 49 percent share represented enough foreign involvement that the Mexican government required lengthy approval procedures. In line with standard regulations, McDonald's was obliged to use only Mexican-produced beef, buns and potatoes.

The only U.S.-produced goods used in the restaurant are some cooking equipment, McDonald's corporate spokesman Robert Keyser said in a telephone interview. "Decisions are made in Mexico," Keyser said.

The opening of the restaurant on Oct. 29 at first appeared to be a triumph. Cars backed up in lines of a mile and longer on the Periphery highway, or beltway, where the McDonald's is located on Mexico City's southern outskirts. Sales were estimated at about $10,000 a day.

After five weeks, however, operation ceased when more than a hundred union activists appeared one day and succeeded in forcing a strike by occupying the driveway and Ronald McDonald playground.

The union militants did not have any visible support from the restaurant's 210 employes when they staged the takeover, according to two employes, a guard at the restaurant, local press accounts and company officials. The pickets, who are now keeping a 24-hour vigil at the restaurant, admitted freely that they work at other restaurants.

The formal objection of the National Union of Restaurant, Hotel, Cantina, Sports Center, Tourism and Allied and Related Workers, which claims to have 85,000 members, is to McDonald's hiring of exclusively part-time workers on individual contracts. That is a novelty in the Mexican industry, where even most hamburger chain workers have standard union contracts guaranteeing an eight-hour day.

One picket said that the union was staging the strike to protect its "source of work."

As a member of the nation's main labor congress, the union is affiliated with the ruling Institutional Revolutionary Party.

But union officials and pickets also used nationalist and even racial terms to describe their cause. They contrasted their Indian appearance and working-class backgrounds with the more European physical characteristics and middle-class origins of both workers and customers at McDonald's.

"We are of this race. You are North Americans, Germans," picketer Antonio Hernandez, 34, explained to three tall U.S. reporters, including two blonds. The restaurant "is in Mexico, so it should hire Mexicans," he said.

Union strike coordinator Sergio Reyes stressed the union's economic reasons for staging the takeover but admitted that there was some class and racial jealousy of the students and housewives employed by McDonald's. "They had college, money and cars," Reyes said. "We are morenos browns , a little ugly."

A lawyer representing McDonld's, Jorge de Regil, dismissed the suggestion that the company was hiring exclusively middle-class or light-skinned persons. "We are not operating a beauty salon," de Regil said.

The union appears to have slim chances of winning a vote, scheduled for Friday, in which the McDonald's work force will consider selecting a bargaining representative, according to both management and union sources.

But these sources and other observers said that McDonald's may continue to have labor difficulties here, possibly with a second union that would seek again to organize the workers. The company is waiting to see if it has troubles at Mexico's second McDonald's, which opened Monday in the northern industrial city of Monterrey.

Union official Reyes conceded that the McDonald's workers were earning more an hour than the standard minimum wage earned under the union's contracts of $3.35 for an eight-hour day.