Protests flared in parts of the United States against stay-at-home orders while governors disputed President Donald Trump's claim they have enough tests for the coronavirus and should quickly reopen their economies.

An estimated 2,500 people rallied at the Washington state capitol in Olympia on Sunday to show their opposition to Democratic Governor Jay Inslee's stay-at-home order, in defiance of a ban on gatherings of 50 or more people.

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Despite pleas from rally organisers to wear face coverings or masks as recommended by public health authorities, many did not.

"Shutting down businesses by picking winners and losers in which there are essential and non-essential are violations of the state and federal constitution," rally organiser Tyler Miller, 39, an engineer from Bremerton, Washington, told Reuters news agency.

“You go to work. Why can’t I go to work!”

This is getting ugly. Anti-lockdown protesters berate nurses blocking them in the street in Denver.

Vis: @marczenn pic.twitter.com/xPrqo5UjFQ — Ashlee Mullany (@AshleeMullany) April 20, 2020

In Denver, hundreds of people gathered at the state capitol to demand the end to Colorado's shutdown. As protesters clogged streets with cars, healthcare workers in scrubs and face masks stood at intersections in a counter protest.

The medical workers were greeted with insults, honking horns, and placards waved by hundreds of motorists saying "End the virus, not the economy" and "Fear is the real virus".

Police were called in after several motorists threatened to run the healthcare workers over.

"You can go to work why can't I go to work?" one woman asked.

Many people expressed concerns that their personal freedoms and constitutional rights were being curtailed in the fight to contain the pandemic.

Stay-at-home measures, which experts say are essential to slow the spread of the virus, have battered the US economy and more than 22 million Americans have filed for unemployment benefits in the past month.

Shutdowns have disrupted economic, social, cultural and religious life and plunged the world into an economic slump unseen since the Great Depression in the 1930s.

The International Monetary Fund expects the global economy to contract three percent this year. Tens of millions of workers have lost their jobs and millions more fear they will be next.

The protesters in Olympia, Washington defied a ban on gatherings of 50 people or more [Lindsey Wasson/Reuters]

Demonstrations to demand an end to lockdown measures have previously erupted in cities in Texas, Wisconsin, and the capitals of Ohio, Minnesota, Michigan and Virginia.

"These people love our country," Trump, who has touted a thriving economy as the best case for his re-election in November, told a briefing in Washington, DC. "They want to get back to work."

Most Americans, by two-to-one, disagree with the protesters, however. A new Pew survey found most were more concerned about ending home confinement too soon rather than too late.

The US has by far the world's largest number of confirmed coronavirus cases, with more than 750,000 diagnosed with the disease and more than 40,500 deaths.

New York has borne the brunt of the virus, which has killed more than 18,000 people in the state, according to data from Johns Hopkins University. On Sunday, there were 507 new deaths from COVID-19, the respiratory illness caused by the coronavirus, down from a high of more than 700 a day.

Hospitalisations also continued to decline to 16,000 from a high of 18,000, and the number of patients being kept alive by ventilators also fell.

The region of Maryland, Virginia and Washington, DC is still seeing increasing cases. New Jersey reported on Sunday that its new cases rose by nearly 3,900, the most in more than two weeks. The cities in Boston and Chicago are also emerging hotspots with recent surges in cases and deaths.

Trump's guidelines to reopen the economy recommend a state record 14 days of declining case numbers before gradually lifting restrictions. Yet the Republican president appeared to encourage protesters who want the measures removed sooner with a series of Twitter posts on Friday calling for them to "LIBERATE" Michigan, Minnesota and Virginia, all run by Democratic governors.

Washington's Inslee stepped up his attacks on Trump's call to "liberate" states, saying the president was encouraging people to violate state laws on self-isolating.

"These orders actually are the law of these states," he said. "To have an American president encourage people to violate the law, I can't remember any time during my time in America where we have seen such a thing."

Virus testing rates have also emerged as a point of contention between Trump and state governors.

Experts also say extensive testing is crucial to a safe reopening of the economy, but some state governors said testing capacity fell far below the levels needed to avoid sparking new outbreaks of the virus.

During a CNN interview, Republican Governor Larry Hogan of Maryland said claims by Trump and Vice President Mike Pence that states have plenty of tests were "just absolutely false".

Democratic Governor Ralph Northam of Virginia said the idea there were enough tests was "delusional".

"Our president, obviously, has been unable to deliver on tests. Now he has chosen to focus on protests," Northam told CNN's State of the Union.

The governors of Michigan and Ohio said they could double or triple their testing capacity if the federal government helped them acquire more swabs and reagents, chemicals needed as part of the testing process.

In New York, Governor Andrew Cuomo urged residents to continue social distancing.

"If the data holds and if this trend holds, we are past the high point and all indications at this point are that we are on a descent," he said, referring to declining death and hospitalisation rates.

"Whether or not the descent continues depends on what we do, but right now we're on the descent," he said, adding that "it's no time to get cocky and it's no time to get arrogant."

Several states, including Ohio, Texas and Florida, have said they aim to reopen parts of their economies, perhaps by May 1 or even sooner.

US legislators are nearing an agreement on approving extra money to help small businesses hurt by the coronavirus pandemic and could seal a deal as early as Sunday, congressional and Trump administration officials said.

Congress established the programme last month as part of a $2.3 trillion coronavirus economic relief plan, but it has already run out of money.