The Federal Reserve Board, concerned about the slide in the dollar and potential inflationary pressures in the economy, yesterday raised the discount rate from 5 1/2 percent to 6 percent, its first increase in three years.

Economists and market analysts said they had expected the Fed to increase the discount rate soon, but the timing of the increase near the Labor Day weekend came as somewhat of a surprise. Some analysts speculated that the increase may forestall a further slide in the value of the dollar before the government releases monthly trade figures next Friday.

Some financial experts predicted that the Fed would have to push rates even higher to help stabilize the dollar. The discount rate is the rate the Federal Reserve charges its member banks for loans.

In an immediate reaction to the rise in the discount rate, most of the nation's major banks raised their prime rates from 8 1/4 percent to 8 3/4 percent. The prime rate is used by commercial banks as the basis for determining interest rates on business and consumer loans.

As a result of yesterday's Federal Reserve action, increased interest rates are expected for personal, small business, home equity and some home mortgage loans. {Details on Page D10.}

The Reagan administration quickly endorsed the increase. "The administration concurs with the action of the Federal Reserve in raising the discount rate in order to deal with potential inflationary pressures," a Treasury Department spokesman said.

White House spokesman Marlin Fitzwater added that the discount rate increase "should be helpful" in maintaining the ongoing economic recovery without rekindling inflation.

The recent slide in the dollar could put upward pressure on prices because a weaker dollar makes imports more expensive and often encourages price increases on domestic goods as well. Low unemployment rates also put pressure on wages as labor shortages occur.

The Veterans Administration, reflecting recent increases in other market rates, yesterday announced it would raise its basic home loan mortgage rate from 10 percent to 10 1/2 percent, effective Tuesday.

The change in the discount rate, which came on the same day that the Labor Department reported that unemployment held at 6 percent in August, was widely interpreted as signaling that new Federal Reserve Board Chairman Alan Greenspan would try to hold inflation in check, as did his predecessor, Paul A. Volcker.

"The move now -- and it's an early move -- clearly suggests that Greenspan is concerned about inflation and is an inflation fighter," said Allen Sinai, chief economist with Shearson Lehman Bros.

"It's a very clear indication that Greenspan is following Volcker's line," said Stephen Marris, a senior fellow at the Institute for International Economics. "He is prepared to use interest rates to defend the dollar."

On Wall Street, stock prices slumped and the Dow Jones industrial average fell 38.11 points. Other stock indices also posted declines {details on Page D10}.

The dollar was bolstered by the rate increase, rising against the yen and the West German mark. The dollar began its most recent slump after it became clear that the U.S. trade deficit in June was $15.7 billion -- larger than expected.

Prices on 30-year Treasury bonds, which are sensitive to changes in interest rates, were depressed, dropping about $2.50 per $1,000 in face value. That decrease added to declines that have been piling up since Tuesday. Yields rose to about 9.46 percent, compared with 9.44 percent on Thursday.

Yields on three-month to one-year Treasury bills also increased.

Bond rates have been pushed up in recent days to account for the inflationary danger that comes with real, and anticipated, declines in the dollar.

"The market's perception was very widespread that this is the first of many discount rate increases that are forthcoming," said Robert Brusca, chief economist at Nikko Securities Inc.

The discount rate had stood at 5 1/2 percent since Aug. 21, 1986, when it was lowered from 6 percent.

The Fed vote for the current increase was 4-0, with governors Martha R. Seger and H. Robert Heller absent for the vote. There is one vacancy on the seven-member board.

The Fed said its decision to increase the rate "reflects the intent of the Federal Reserve to deal effectively and in a timely way with potentially inflationary pressures."

Most economists applauded the move and expected that more might follow. "I consider this a first step in steps to get short-term interest rates up," said Robert Jenetski, chief economist for Harris Bank in Chicago. "The economy has been much stronger than people believe it to be."

Jenetski pointed to increases in commodity and gold prices, as well as strong rates of employment. "They may have to raise interest rates more than anyone would like to see."

Sinai, however, insisted that the increase "doesn't have to be a prelude to higher interest rates."

"This was designed to prick the balloon of inflation and inflationary expectations rather than to slow down the economy," Sinai said. "The jury is out on future interest rate hikes."

Jerry Jasinowski, chief economist for the National Association of Manufacturers, said the Fed principally is concerned about stabilizing the dollar and the bond markets. "The Fed increasingly is a hostage to the dollar and is concerned it could go into a more serious downward spiral," he said.

Some economists also regarded the move as a politically astute one for Greenspan, who is scheduled to meet his international peers at a meeting of the Bank for International Settlements on Monday in Basel, Switzerland.

"Consider it his calling card for the meeting," said David A. Wyss, chief financial economist for Data Resources. "He may have been criticized for not holding the dollar up, and it will put more pressure on central banks to help."