Bond prices, which had been rallying in recent weeks, nosedived today as the Federal Reserve Board took steps to drain credit from the banking system and the Treasury announced plans to borrow $4.75 billion.

Separately, a Treasury Department spokesman confirmed that Secretary Donald T. Regan has invited a group of Wall Street executives and chief officers of major corporations from throughout the nation to meetings in Washington on Wednesday and Thursday.

The purpose of the sessions wasn't disclosed, but President Reagan and his top spokesmen have been pointedly critical of the investment and business community in recent weeks for causing weakness in financial markets and for lack of tangible support of the administration's economic program. Fewer than 20 persons have been invited to each of the two sessions.

The continued market weakness was evident again today. "We were clobbered," Bernard Harmon of the big brokerage firm Drexel Burnham Lambert Inc. said about the bond market. He said most bonds lost between $12.50 and $15 for each $1,000 of their face value. Stock prices also declined slightly today (Story on Page D9).

Bonds are long-term securities that have fixed interest rates and are sold by governments and businesses. Because the interest return is fixed, prices of the bonds change depending on interest rates. When rates rise, bond prices fall and vice versa.

Even after today's losses, which were a continuation of a price decline that began late Monday afternoon, the average bond is selling for $40 to $50 more than it was at the beginning of the month, when the beleaguered bond market was at a record low.

Harmon said the Federal Reserve, the nation's central bank and keeper of monetary policy, entered the market to drain reserves from the banking system when the so-called federal funds rate was about 14 1/2 percent. The federal funds rate is the interest banks charge one another for overnight loans of excess funds. The rate can be affected strongly by the open market operations of the central bank in which it buys or sells government securities in an attempt to keep the growth of the money supply on a course the Fed deems proper.

When the Federal Reserve drained funds from the banking system, Harmon said, "it was an indication that the federal funds rate is not going to go a lot lower" for a while. Although the Federal Reserve said it does not conduct its monetary policy with much of an eye toward short-term interest rates, most buyers and sellers of bonds carefully watch the federal funds rate.

After the Fed entered the market, Harmon said, many nervous bond holders who were uncomfortable hanging on to their securities but reluctant to sell as long as bond prices were rising, sold bonds and helped drive their price down.

Investors also were apprehensive about the Treasury's borrowings, even though the announcement was expected, because Treasury borrowings can squeeze other investors out of the market and drive up interest rates.

The key Treasury bond that matures in 2011 ended the day at about 96 3/4. That means an investor would have to pay $967.50 to buy a $1,000 bond. The bond early this month was as low as $925 and last week traded about $990.

In Washington, Regan argued that financial markets are, in the short term, putting what he called misplaced emphasis on weekly money-supply statistics, and, in the long run, expressing uncertainty that the federal government will stick to the Reagan economic program.