Among the dozens of leaflets left in mailboxes during the campaign leading up to today's national election, there is one with a particularly pointed message: Keep Brussels from becoming a Chicago or Beirut.

Although it might surprise Americans to learn that Chicago still has a negative reputation long after the end of its gangland era, both the Midwestern city and the Lebanese capital are code words here for urban violence and disorder.

The real concerns of the two members of parliament distributing the leaflets in their reelection campaign, however, are Belgium's 200,000 Moslem foreign workers and family members, not the threat of mobster-style crime.

Claiming that immigrants are "to a great degree responsible for the insecurity" in Brussels, the leaflet says it is necessary to reduce their presence "with all means" available.

"Arm us" with your votes, the leaflet asks, so that "finally, real solutions" can be found to the "real problem of immigration."

The popularity of anti-immigrant candidates in Belgium has underlined the potency of the issue in Western European nations with large foreign worker populations. Immigrant groups and their supporters say persistent high unemployment, overburdened welfare systems and the perception of rising crime have created a new mood of intolerance in which foreign workers are made the scapegoats for economic and social ills.

The immigration issue in Belgium is especially acute in Brussels, where in some districts foreign workers and their families make up almost 50 percent of the population. The Moslem immigrants, mostly Turks and Moroccans, were actively sought by the Belgian government during the 1960s, when there was a shortage of manpower for menial jobs.

Today, the Moslems face the same economic slump that has thrown thousands of Belgians out of work, but the immigrants are at a greater disadvantage because of a lack of education and cultural differences. They live side by side with equally poor Belgians in deteriorating neighborhoods described as ghettos by some local politicians.

Confrontations between police and young immigrants in these neighborhoods are not uncommon. Late last month, four police officers and two Moroccans were seriously injured in the Schaerbeek district during a melee that began, police said, when the officers tried to check a youth's identity card.

In the days before today's election, supporters of right-wing candidates were busy in the districts shared by immigrants and Belgians, putting up posters promising "security for Brussels."

Among the measures supported by these candidates is expulsion of immigrants who have been unemployed for a long period, along with those convicted of crimes. The candidates also want steps to encourage private business to give priority to hiring Belgian citizens.

According to Guy Cudell, a Socialist senator, simplistic slogans about immigration are poisoning the electoral campaign.

The immigration issue is expected to boost the chances of one member of the government coalition, the French-speaking, right-of-center Liberal Reform Party.

One recent poll showed the party, which advocates breaking up the neighborhoods of "immigrants difficult to integrate" into society, gaining two seats from the capital.

The center-right government of Prime Minister Wilfried Martens has already responded to the preoccupation with immigrants, approving legislation last year giving individual districts of Brussels the power to reject the applications of non-European Community immigrants who wish to live within their borders, if it is considered in the public interest. The law was sponsored by a Liberal Reform leader, Justice Minister and Deputy Prime Minister Jean Gol.

Leaders of Martens' Christian People's Party, which has taken a more moderate stand on the immigrant issue, said pressure from the Liberal Reform Party to toughen government policies could be a source of tension if the present coalition is returned to office.

The Belgian law on immigrants, which EC officials believe is unique within the 10-nation community, has been assailed by immigrant groups, who said that immigration virtually ended several years ago. The main effect of the law will be to complicate the entry of political refugees and limit the freedom of movement of immigrants already in the country, the groups said.

Recounting acts of violence against immigrants, their mosques and the cafes where they gather, the groups warn that the law, together with new government financial incentives to encourage foreign workers to return to their countries of origin, will encourage prejudice.

"We think it the government policy is the first step toward a form of apartheid," said Yvonne Jospa, a member of the Brussels-based Movement Against Racism, Anti-Semitism and Xenophobia known as MRAX.

While support for the anti-immigration law spans the political spectrum, the opposition Socialists emphasize that the government should recognize that most immigrants do not wish to return to their home countries, and should call for efforts to improve housing and education for foreign workers.

"It's certainly necessary to halt immigration," Cudell said. "But our task is to prepare this city for pluralism. How can you speak of Brussels as the 'capital of Europe' as the home of the EC headquarters without foreigners?"