The government reported yesterday the most dramatic sign yet that the industrial boom in the economy has begun to abate : New orders for durable goods, a key measure of business activity, plunged abruptly in April.

Commerce Department figures showed new orders plummeted $7.31 billion, or 8.7 percent, to $77.03 billion last month-their steepest drop in more than 11 years. The decline followed a surge of 2.8 percent in March.

Although the April drop was a sizable one, analysts cautioned against reading too much into the figures. The consensus was the decline reflected a definite slowing in business activity, but probably not an immediate recession.

In a way, the dropoff was considered welcome. Many economists had feared the frenetic pace of business activity earlier this year would heighten inflationary pressures and bring on a deeper recession than expected.

Nevertheless, the April decline was steep enough that some analysts suggested the economy may be entering a downturn. William Cox, the department's deputy chief economist, conceded the figures were "a matter of concern."

Yesterday's report came as, separately, the Federal Reserve Board's policymaking Open Market Committee met to review money and credit policies, but apparently did not move immediately to raise or lower interest rates.

Wall Street analysts said while the key "federal funds" rate dipped to just above 10 percent, from 10.25 percent before, there was no indication of any shift in policy. The rate is the interest on overnight loans to banks.

However, economists said yesterday's market action was far from conclusive. The Open Market Committee's decisions often are not made known for several days, and yesterday the Fed was engaged in technical buying actions.