A new surge of imported goods is displacing domestic production and holding the nation's economic growth to an unexpectedly low 2.1 percent annual rate in the first quarter, the Commerce Department reported yesterday.

The slow rate of increase in the gross national product, adjusted for inflation, is only about half as large as that expected by many forecasters. It is also down from a revised 4.3 percent figure for the fourth quarter of 1984. The fourth quarter rate was estimated earlier at 4.9 percent.

Consumer spending is rising strongly enough this quarter that overall demand for goods and services is going up considerably faster than GNP, which is a measure of production. The added demand, however, is being satisfied by imported goods and services rather than those made in the United States.

News of the low figure contributed to a large drop in the value of the dollar on foreign exchange markets yesterday, traders said. The dollar fell 1.8 percent against the West German mark and the French franc and a whopping 3.85 percent against the British pound.

Slower economic growth in the United States could mean that interest rates and corporate profits will be lower than anticipated, and thus lower the demand for dollars by foreigners intending to invest in the United States.

The Commerce Department also said that inflation, as measured by the GNP implicit price deflator, is jumping from a 2.8 percent pace in the fourth quarter to a 5.4 percent rate this quarter.

Much of the higher rate is because of a change in the composition of output -- including a large rise in energy purchases, which still carry relatively high prices -- rather than just a direct increase in prices.

A separate price index that is not affected by such changes in the mix of goods and services actually produced rose at a 4.1 percent rate, up from 3.6 percent in the previous three months.

The White House issued a statement declaring, "We continue to believe that the economy is on a path of steady growth with low inflation." It pointed to the small increases in producer prices in January and February and in the consumer price index for January as evidence that inflation was remaining low.

The February CPI will be released today, with analysts expecting an increase of about 0.3 percent, up slightly from the January rate.

The GNP number reported yesterday for the current quarter -- the department's so-called flash estimates -- is based on data from only one or two months and projections of the missing figures by the Bureau of Economic Analysis. The estimates are subject to substantial revision as more complete data become available.

The flash estimate for first-quarter real GNP, coupled with the 1.6 percent increase in the third quarter of 1984 and the revised fourth quarter figure, means that the economy has grown at an average rate of only 2.7 percent over the last nine months.

According to most economists, that is not fast enough to reduce the nation's civilian unemployment rate. The unemployment rate was 7.3 percent in February.

Most economic forecasters have been expecting real GNP to rise at about a 3.5 percent or 4 percent pace during this year. However, some of them have warned that variations in foreign trade could affect the quarter-to-quarter rates of increase in GNP reported by the Commerce Department.

One such economist is Alan Greenspan of Townsend-Greenspan & Co., who said, "The problem really gets down to a judgment of the March trade figures, about which we have no information . . . We need another month's worth of data. There is nothing in this report that changes our view of what is going on. It is very tough to read what is going on in the GNP with the trade accounts so unstable."

However, some other forecasters have been toning down substantially their expectations for the economy this year. Among them is Wharton Econometric Forecasting Associates, whose latest prediction was for a 4.9 percent gain this quarter, dropping to 2.9 percent in the second and to 1.7 percent in the third and fourth quarters.

After yesterday's flash estimates were released, Wharton economist David Berson said the report probably signals that the economy is entering a "growth recession" -- with rising unemployment -- sooner than had been expected. But Berson predicted the economy will show renewed strength in the final three months of the year, assuming that the Federal Reserve Board will loosen its restraints on the money supply.

"Growth will rebound because the Fed will have to respond to prevent a growth recession from becoming a real recession," Berson said.

Federal Reserve policy-makers will meet next week to chart a short-term course for monetary policy. Analysts said that the apparently weak economic growth this quarter probably has removed any likelihood that the policy-makers will choose to tighten policy to slow down growth of the money supply.

Many of the monthly statistics released so far for January and February have painted a somewhat confusing economic picture. Industrial production fell 0.5 percent in February, while retail sales rose a strong 1.4 percent. Personal income increased only 0.3 percent last month, and would have been up about 0.1 percent in January except for a change in the timing of a payment of military retirement benefits.

As a result of the slower rise in income and continued strong consumer buying, the personal savings rate fell to 5.2 percent of disposable income in February. If income gains do not pick up, spending increases undoubtedly would slow, analysts said.

The Commerce Department said business investment in new plants and equipment and investment in new residences will be up little from the fourth quarter