WASHINGTON — The number of illegal immigrants entering the United States has plunged by almost two-thirds in the past decade, a dramatic shift after years of growth in the population, according to a new report by the Pew Hispanic Center.

Between 2000 and 2005, an average of 850,000 people a year entered the United States without authorization, according to the report released yesterday. As the economy plunged into recession between 2007 and 2009, that number fell to 300,000.

The sharp drop-off has contributed to an 8 percent decrease in the estimated number of illegal immigrants living in the United States, from a peak of 12 million in 2007 to 11.1 million in 2009, the report said. Of the 11.1 million, 8.9 million came from Mexico and other parts of Latin America. Virginia, Florida, and Nevada were among the states with steepest declines in their populations of illegal immigrants.

An estimated 160,000 illegal immigrants were living in Massachusetts in 2009, about 2.5 percent of the population, according to the center. That number is down 20 percent from 200,000 in 2005.

The new figures come amid a heated national debate over efforts by Arizona and other jurisdictions to identify people who are here illegally and push to have them deported.

Douglas Massey, a Princeton University sociologist who studies migration, said the recession and lack of jobs are major factors in the decline of those entering the country illegally.

The unemployment rate for illegal immigrants is 10.4 percent higher than that of either US-born residents or legal immigrants, the Pew report said.

Massey said other likely reasons for the decline include an increase in law enforcement and deportations and enactment of stricter legislation against illegal immigrants. He also pointed to more guest-worker spots — from 104,000 in 2000 to 302,000 in 2009 — allowing more immigrants to come to the United States legally.

“Life’s gotten pretty miserable for immigrants in the United States,’’ Massey said, noting that even for legal immigrants, many of whom have relatives who are unauthorized, the increased scrutiny has been stressful.

Still, the flow of legal immigrants into the United States has increased slightly over the past decade, according to the report, noting that the trends have reduced the percentage of immigrants who are here illegally, from 31 percent of all immigrants in 2007 to 29 percent in 2009.

Although an earlier Pew study pointed to signs of fewer illegal immigrants in recent years, the new report reveals the first statistically significant reversal in the growth of the population in the past 20 years, said Jeffrey Passel, a senior demographer at Pew.

Passel, an author of the new report, noted that in recent years, illegal entry has become more expensive and more dangerous.

“We know that it’s harder to sneak across the border than it was four or five years ago, and especially than it was 10 or 15 years ago,’’ Passel said.

“Virtually everyone who sneaks across the border uses a [guide] now, and the cost has gone up. The increase of the border patrol around cities and ports of entry has pushed the flows across the border into more remote places.’’

The report’s findings were hailed by Mark Krikorian, director of the Center for Immigration Studies, which advocates tighter immigration controls.

The figures contradict “the idea that the only options before us are mass expulsions or mass amnesty,’’ Krikorian said. “This finding points to the middle way, of a consistent decrease of the illegal population over time through enforcement.’’

Large declines in illegal immigrants in Florida and Nevada are likely because of the mortgage and foreclosure crises and the loss of thousands of construction jobs, which immigrants often fill, Passel and Massey said. Florida’s illegal immigrant population fell by 375,000, to an estimated 675,000, between 2008 and 2009, and Nevada’s decreased by 50,000, to an estimated 180,000, during that period.

The nationwide trajectory will probably depend on the strength of the country’s economic recovery and the level of enforcement of immigration laws, Passel said.

“In the past the flows have moved in line with the state of the US economy,’’ he said. “But we have stepped up enforcement right now. Right now, both are working in the same direction. If the economy turns around and enforcement is increased, we don’t know.’’

© Copyright 2010 Globe Newspaper Company.