With health authorities and many governors warning that far more testing is needed before the US economy can be safely reopened, New York state launched the nation's most ambitious effort yet to screen the general population for exposure to the virus.

At the same time, researchers began an effort to test residents of an entire town near San Francisco for antibodies, while a broader sampling in Los Angeles County suggested 40 times as many people were infected there as the number of cases previously documented.

At least three more governors, nevertheless, moved to loosen restrictions on commerce in their states.

South Carolina Governor Henry McMaster, a Republican, signed an order allowing retail shops and department stores to resume business on Tuesday, with limits on how many customers are allowed in. Colorado Governor Jared Polis, a Democrat, said he would permit reopening of hair salons, child care centers and real estate offices, also subject to social-distancing measures, starting next week.

And Georgia Governor Brian Kemp, a Republican whose state reported over 1,200 new infections and a spike in deaths on Monday, announced that gyms, hair salons, bowling alleys, tattoo parlors and massage parlors could reopen on Friday, followed on Monday by movie theaters and restaurants.

Stay-at-home orders and widespread business closures imposed in most states to slow the spread of the virus have stifled the US economy and thrown at least 22 million people out of work, a level of unemployment not seen since the Great Depression of the 1930s.

The economic damage has led to increasing agitation for relaxing social-distancing restrictions, especially as the rate of coronavirus hospitalizations and other indicators of the outbreak's severity have begun to level off in recent days.

In Pennsylvania, where Democratic Governor Tom Wolf has pledged to veto a bill in the Republican-led General Assembly that would force him to reopen some businesses, several hundred demonstrators, some in cars with horns blaring, rallied in the state capital, Harrisburg.

Cynicism vs. fear 

Many protesters were skeptical about the actual scale of the pandemic, accusing political leaders of over-reaching.

"All the projections were wrong, but we are still telling people to stay home and businesses to close. This is not quarantine, this is tyranny," said Mark Cooper, a 61-year-old retired truck driver.

Others portrayed the stay-at-home measures as essential to save lives. Yetta Timothy, who was part of a counter-protest in Harrisburg, said the nursing home where she worked had lost an untold number of patients.

"They are dying everyday," said the 43-year-old nurse, crying and holding a sign that read: "My life is on the line."

"I just can't believe all of this is happening, that they want to go back to work," she said.

Protests demanding an end to stay-at-home restrictions also were held in Pittsburgh and in the Connecticut state capital, Hartford.

Monday's demonstrations, like those in the capitals of several other states in recent days, including Michigan, Minnesota, Virginia, Maryland, Washington state and Colorado, drew large contingents of self-identified supporters of President Donald Trump.

Expressing sympathy for the protesters, Trump lashed out on Twitter last week at Democratic governors in three electoral swing states, saying their stay-at-home orders had gone too far.

One governor Trump targeted, Gretchen Whitmer of Michigan, said she appealed to Vice President Mike Pence during a gubernatorial conference call on Monday for federal officials to speak out in support of social-distancing restrictions imposed by state leaders.

Whitmer told reporters after the call that Pence, who is leading the Trump administration's pandemic response, vowed on the call to do as she requested.

Warnings of resurgence 

Medical professionals on the front lines of the battle to curb the pandemic, which erupted in China late last year, have said the United States could face a second and even deadlier wave of infections if the lockdowns end prematurely.

The United States has by far the world's largest number of confirmed cases of COVID-19, the highly contagious lung disease caused by the coronavirus, with more than 778,000 known infections and over 42,300 deaths, nearly half of them in the state of New York, according to a Reuters tally.

Dr. Anthony Fauci, head of the National Institute of Allergy and Infectious Diseases, told ABC News there would be no real economic recovery until authorities got the virus under control and jumping the gun could lead to a big spike in cases.

"It's going to backfire, that's the problem," he said.

New York Governor Andrew Cuomo said testing has to get up to scale before reopening, adding that he was aware some people were unhappy they had to wear masks or engage in social distancing. "It's not a question of happy - it's a question of life and death." .

Researchers on Monday began conducting antibody blood tests on New York state residents to obtain a baseline of how many may have actually been exposed to the novel coronavirus. Cuomo said specimens would be collected from a random sample of 2,000 people a day in what he called the most aggressive such project to date in the United States.