The proportion of Americans living in poverty dropped sharply in 1984 to 14.4 percent of the population, almost a full point lower than the 15.3 percent recorded in 1983 and the first decline after five years of steep increases, the Census Bureau reported yesterday.

The bureau also said median family income rose 3.3 percent to $26,433 in 1984 after accounting for inflation, the fastest rate of increase since 1972. Rising personal income plus a drop in unemployment led to the decline in poverty.

Officials and economists said the drop in poverty resulted primarily from the strong economic recovery of 1983-84 after two years of recession, plus a fall in average unemployment rates from 9.6 percent in 1983 to 7.5 percent in 1984.

The Reagan administration quickly hailed the news. In a statement released in Santa Barbara, Calif., where the president is vacationing, he said the numbers are "further proof that the greatest enemy of poverty is the free enterprise system." He said the fight against poverty is not over but that the nation "is once again headed in the right direction."

Whether the decline will continue if the economy slows is uncertain.

The previous five-year increase in the poverty rate, from 11.4 percent in 1978 to 15.3 percent in 1983, is usually attributed to slow economic growth, inflation, recession and high unemployment, plus such demographic factors as the rise in the one-parent female-headed family. Many economists think that cuts in government aid to the poor contributed to increased poverty.

According to the Census Bureau, the total number of people in poverty in 1984 was 33,700,000, compared with 35,515,000 in 1983.

The new figures, coming after several years in which the Reagan administration was widely blamed for rising poverty, were hailed by White House officials as proof that President Reagan's economic policies have worked.

"This is unambiguous good news," said Patrick J. Buchanan, White House director of communications. He said the downturn in poverty shows that robust economic growth does more to cut poverty than anything else.

"It is not only a triumph for the Reagan policies but a triumph for the Reagan philosophy . . . . It is undeniable that the president's policies deserve enormous credit," said Buchanan, citing the 1981 tax cut and efforts to combat inflation.

Democrats and liberal analysts were far less upbeat in their assessment.

The Center on Budget and Policy Priorities, a research and analysis organization that has been sharply critical of Reagan's policies, said that at 14.4 percent the 1984 figure is still nearly three points higher than in 1976-77 when unemployment was at about the same level as now. The poverty rate is also higher than when Reagan completed his first year in office; it was 14 percent in 1981.

Moreover, center officials said, on the basis of current economic projections, "We expect no further significant drop in 1985." They said some administration officials and many economists had expected an even sharper decline in poverty.

One reason that did not occur is that the economic recovery "has not been evenly shared. The census data show that the gap between rich and poor is now wider than at any time in the last 40 years," they said in a statement.

In 1983, the statistics show, the lowest two-fifths of families got 15.8 percent of all income, and the highest two-fifths got 67.1 percent. In 1984, the lowest two-fifths got less, 15.7 percent, while the highest two-fifths got 67.3 percent.

The center cited budget cuts in programs for the poor and failure to raise the minimum wage as contributing factors. The center also noted that for black children under 6, the poverty rate in 1984 was 51.1 percent, the highest rate recorded since calculations for this group started in 1970.

Reps. Charles B. Rangel (D-N.Y.) and Robert T. Matsui (D-Calif.) said some of the most disadvantaged groups have advanced little. "Real median income is still slightly below the 1980 level and the poverty rate remains 1.4 percentage points higher than the 1980 level 13 percent . Since 1980 the rich have gotten richer and the poor poorer," they said in a joint statement.

They said median income of the bottom 40 percent from 1980 to 1984 declined from $12,966 to $12,489 in constant dollars while median for the top 40 percent of families increased from $43,531 to $45,300.

The official government poverty line in 1984 was $10,609 for a family of four and at different levels for other size families. It is based on cash income only. Noncash benefits such as Medicaid, Medicare, food stamps, subsidized housing and school lunches do not count.

The bureau estimated that if all these benefits were counted the proportion of people in poverty would be 9.7 percent (counting the benefits at market value) to 12.2 percent (counting them at less than market value, which many economists think that is the proper measurement).

Although the poverty rate dropped for most groups, tremendous variations remained among demographic categories:

*By race, the poverty rate dropped from 12.2 percent in 1983 to 11.5 percent in 1984 for whites, and from 35.7 percent to 33.8 percent for blacks, but rose from 28.1 percent to 28.4 percent for Hispanics.

*For people 65 and over, it dropped from 14.2 percent to 12.4 percent, the lowest figure ever recorded, reflecting control over inflation -- which particularly affects the elderly because many of them are on fixed incomes, increases in those receiving private pensions and the Social Security cost-of-living increase in January 1984.

*For children under 18, the poverty rate dropped from 22.2 percent to 21.3 percent, but the improvement was all among whites. For black children, it remained the same at 46.5 percent. For Hispanics, it rose over a point to 39 percent.

*For one-parent female-headed families, it dropped from 36.1 percent to 34.5 percent.