Black civil rights leader Ralph David Abernathy endorsed Ronald Reagan for president today and denounced Jimmy Carter for making "empty promises" to the nation's poor.

"I campaigned for Gov. Carter four years ago, but President Carter has not kept his campaign promises," Abernathy told an audience at an inner-city Detroit church as Reagan and his running mate looked on. "Inflation has increased, unemployment has increased, interest rates have increased. Poor black people cannot make it under this system. And we don't need this doctor anymore, because we as patients are getting sicker."

The endorsement was highly coveted by the Reagan campaign, which has lacked the support of any major figure in the black community. If Carter receives more than 90 percent of the black vote, it could tip the election to him in strategic industrial states, in the opinion of strategists in both the Carter and Reagan camps.

"I think it's magnificent," Reagan said of the Abernathy endorsement as he left an audience of about 100 at the church. "It's a great help . . . I just didn't realize such a thing could happen. I am overwhelmed."

Reagan strategists had been working to obtain the endorsement since August, particularly after attempts to gain the backing of the Rev. Jesse Jackson fell through. But Abernathy had made no commitment before a private meeting with Reagan this morning, and the Reagan camp was wary of claiming any endorsement until it was actually made.

Abernathy, who backed Sen. Edward M. Kennedy (D-Mass.) in the Democratic primaries, frequently has been at odds with other civil rights figures, especially Carter supporters Coretta King and Andrew Young.

Abernathy, at one time the righthand man to Martin Luther King Jr., said he had been praying for weeks about what his decision should be. He said that one of his concerns was that the Democratic Party not take black people for granted.

"The Democratic Party in Washington is not going to understand that black people are part of America until we let them know we do have an alternative," Abernathy said. Then he added some harsh words for Carter.

"Jimmy Carter did not do as much for the black people of Georgia as Lester Maddox did," Abernathy said. "Ronald Reagan did a whole lot more for black people and poor people in California than Jimmy Carter did in Georgia. I'm sick and tired of empty promises."

Two civil rights colleagues of Abernathy -- Hosea Williams of the Southern Christian Leadership Conference and E.V. Hill, a Los Angeles minister -- also endorsed Reagan at the church meeting.

[A Washington Post poll of eight large states found Reagan's support among black registered voters ranging from a low of 0 percent in Michigan to 11.8 percent in Illinois, with the overall eight-state average at 5.2 percent. Carter's support ranged from a low of 59.3 percent in Illinois to 88.8 percent in Ohio.]

[The poll also found that in Texas, New York and New Jersey, where Reagan's support among blacks is no better than 2.1 percent, one-fifth to one-quarter of the blacks surveyed said they were undecided. The poll was taken Sept. 26 to Oct. 5 and included 600 registered voters in each state.]