By Lucia Mutikani

WASHINGTON (Reuters) - The number of Americans filing claims for unemployment benefits likely raced to as high as a record 4 million last week as strict measures to contain the coronavirus pandemic ground the country to a sudden halt, unleashing a wave of layoffs.

The weekly jobless claims report from the Labor Department on Thursday is set to offer the clearest evidence yet of the coronavirus' devastating impact on the economy, which has forced the Federal Reserve to take extraordinary steps and set the U.S. Congress racing to assemble a record $2 trillion stimulus package.

Economists say the economy is already in recession and the jobless claims report would offer proof of that.

The weekly claims figures are the most timely labor market indicator. The report on Thursday is set to grab attention on both Wall Street and Main Street after the Trump administration requested that states stop giving daily snapshots of applications for jobless aid.

According to a Reuters survey of economists, initial claims for state unemployment benefits probably surged to a seasonally adjusted 1 million for the week ended March 21, which would far eclipse the previous record of 695,000 set in 1982. Estimates in the survey were as high as 4 million, which would dwarf the 281,000 applications received during the week ended March 14.

"Containment efforts in response to the coronavirus resulted in a very sudden and very dramatic change over just a few days," said Stephen Gallagher, U.S. chief economist at Societe Generale in New York. "Layoffs were part of that change and applicants appear to have flooded state unemployment insurance offices within a very short time-span."

(GRAPHIC: Unemployment benefits claims to surge - https://fingfx.thomsonreuters.com/gfx/mkt/13/3944/3900/Pasted%20Image.jpg)

There were reports of many states saying their employment websites crashed because of heavy traffic.

Governors in at least 18 states, accounting for nearly half the country's population, have ordered residents to stay mostly indoors. "Non-essential" businesses have also been ordered closed. According to economists, a fifth of the workforce is on some form of lockdown.

Economists' collection of raw data from states, industry groups and their own models show an unprecedented jump across all states.

Morgan Stanley is forecasting unadjusted claims for California, one of the regions hardest hard by the respiratory illness called COVID-19 brought on by the coronavirus, to have shot up by 550,000. California Governor Gavin Newsom said earlier this week that new filings for jobless benefits there were running at an average of 106,000 a day in the past week.