EAST BERLIN, NOV. 21 -- Faced with a mushrooming black -- market trade with West Germany, East German Prime Minister Hans Modrow warned today that "unpopular measures" would soon be taken to check the flow of goods and currency across the country's newly opened borders. East Germany "must not become a nation of speculators," Modrow said in remarks at a factory, quoted by the official news agency ADN. "I cannot understand how many citizens are simply throwing away money they worked hard for." In the 12 days since East Germany allowed its citizens to travel freely to the West, more than 7 million have visited West Germany and a lively trade in East German currency and goods has sprung up in West Berlin and other West German cities. East Germans eager to buy Western consumer goods have traded their own currency for rates as low as 20 to one against the West German mark in recent days, far below the official rate of one to one. Also, East Germans, Poles and some Westerners have bought up heavily subsidized goods in East German stores and transporting them to the West for resale. East German officials say this popular entrepreneurship is threatening to deal a serious blow to an already beleaguered economy, worsening shortages and creating powerful inflationary pressures. Modrow indicated that measures to deal with the problem will be announced on Thursday, although he did not specify what these would be. Economists and Western diplomats argue that, short of resealing the border, East Germany will be unable to stop black -- market trade in goods and currency unless it radically alters its system of mandated prices -- removing heavy subsidies for food and other basic items and taking steps to make its currency fully convertible. For now, however, East German officials appear to be planning a series of stopgap administrative measures that may hinder what has been a relatively free flow of traffic across the inter-German border. Diplomats said they expected customs checks may be tightened at the border and that the prices of some heavily subsidized items, such as children's clothing, may be raised or its sale limited. Bishop Gottfried Forck of East Germany's Protestant Church said West Germany should consider imposing its own customs and exchange controls to limit smuggling. "I asked for help to protect East Germany from bleeding to death financially and economically," Forck told reporters after meeting West German Chancellery Minister Rudolf Seiters in East Berlin. The forces driving the black-market trade are easy to see in the stores of East and West Berlin. In the West, huge crowds of East Germans continue to flock to discount clothing and department stores, and many are ready to exchange their own currency at rock-bottom rates to buy the radios, toys, cosmetics, tropical fruit and chocolate unavailable in the East. In the stores of East Berlin, meanwhile, staggering bargains await traders, many from the West or Poland. With East German marks purchased in West Berlin, buyers can get a well-made boy's ski jacket for the equivalent of $1.91, women's leather boots for 86 cents, or a pair of skis for $5. At the entrance to Centrum Markethalle, the city's largest department store, three Polish men were sprawled on the floor at midmorning today surrounded by 11 canvas bags stuffed with goods they had bought in a matter of hours. "Everything is worth buying here," said Piotr, a 25-year-old ambulance driver from the Polish city of Torun. "Everything is a deal." After buying goods in East Germany, many Poles cross to West Berlin and resell them on a huge muddy field that has become known as the "Polish market." The West German marks they earn can be exchanged for more East German money at the black-market rate, or used to buy Western goods for resale in East Germany or Poland. So great has the traffic of Poles across the East-West border in Berlin become that East German border authorities have set up special lines and facilities to handle it, marked with Polish-language signs. The party daily, Neues Deutschland, today published a front-page photograph of police removing goods from a Polish-built Fiat. An accompanying article reported that one Pole had been caught crossing to the West with 220 pounds of hazelnuts, 16 fishing rods, 26 fishing nets and 60 spools of fishing line. Seiters, a top aide to West German chancellor Helmut Kohl, proposed Monday in discussions with Modrow and East German Communist party leader Egon Krenz that the two countries set up a special currency fund of West German marks that would be used to supply East Germans with hard currency for visits and shopping in the West. Bonn sees the proposed fund as substituting for the "welcome money" it now offers to every East German crossing the border once a year. By obliging East Germans to exchange their own marks for the Western currency, West Germany could help stabilize the East German currency and obtain a supply of money it could invest in East Germany. However, Seiters indicated that Bonn was seeking East German concessions in exchange for the currency fund, including the abolition of rules that now force West Germans to change 25 marks a day while visiting the East at the official 1-to-1 rate. Also today, an East German newspaper attacked Krenz, another sign of Krenz's weakening position as a Communist Party congress approaches next month. The state-run National-Zeitung noted that Krenz belonged to the "leadership body that is now in the crossfire of criticism." Referring to Krenz's support for China's suppression of its movement for democracy and alleged involvement in falsifying East German electoral results earlier this year, the paper said "Krenz cannot make up lost ground."