More than three billion people are living under lockdown measures as soaring death tolls in Europe and the US on Wednesday underlined a United Nations warning that the coronavirus pandemic threatens all of humanity.

As the global death toll soared past 20,800, Spain joined Italy in seeing its number of fatalities overtake China, where the virus first emerged just three months ago.

"COVID-19 is threatening the whole of humanity -- and the whole of humanity must fight back," UN Secretary General Antonio Guterres said, launching an appeal for $2 billion to help the world's poor.

"Global action and solidarity are crucial. Individual country responses are not going to be enough."

Markets rebounded after Washington vowed to spend $2 trillion to fend off a US economic collapse, while governments elsewhere were taking ever more dramatic steps to contain the disease.

India's stay-at-home order for its 1.3 billion people is now the biggest, taking the total number of people facing restrictions on their daily lives to more than three billion.

Russia, which announced the death of two patients who tested positive for coronavirus on Wednesday, is expected to follow suit.

President Vladimir Putin declared next week a public holiday and postponed a public vote on controversial constitutional reforms, urging people to follow instructions given by authorities.

In Britain, heir to the throne Prince Charles became the latest high-profile figure to become infected, though he has suffered only mild symptoms.

The G20 major economies will hold an emergency videoconference on Thursday to discuss a global response to the crisis, as will the 27 leaders of the European Union, the outbreak's new epicentre.

China has begun to relax its own draconian restrictions on free movement in the province of Hubei -- the original epicentre of the outbreak -- after the country reported no new cases.

Crowds jammed trains and buses in the province as people took their first opportunity to travel.

But Spain saw the number of deaths surge to 3,434 after 738 people died in the past 24 hours and the government announced a 432-million-euro ($467 million) deal to buy medical supplies from Beijing.

'People dying alone'

At La Paz university hospital in Madrid, accident and emergency nurse Guillen del Barrio sounded bereft as he related what happened overnight.

"It is really hard, we had feverish people for many hours in the waiting room," the 30-year-old told AFP.

"Many of my colleagues were crying because there were people who are dying alone, without seeing their family for the last time."

Spain and Italy were joined by France and six more EU countries in urging Germany and the Netherlands to allow the issue of joint European bonds to cut borrowing costs and stabilise the eurozone economy.

The call is likely to fall on deaf ears when EU leaders talk on Thursday -- with northern members wary of pooling debt with big spenders -- but they will sign off on an "unprecedented" recovery plan.

The death toll in Italy jumped in 24 hours by 683 to 7,503 -- by far the highest of any country.

The number of French deaths was up by 231 on Wednesday to 1,331, and metro and rail services in Paris were cut to a minimum.

Coronavirus cases are also spreading in the Middle East, where Iran's death toll topped 2,000, and in Africa, where Mali declared its first case and several nations announced states of emergency.

And in Japan, which has postponed this year's Olympic Games, Tokyo's governor urged residents to stay home this weekend, warning of a possible "explosion" of the coronavirus.

Jerusalem's Church of the Holy Sepulchre, believed by Christians to house Christ's tomb, was shut as Israel tightened movement restrictions.

The impact of the pandemic is also hitting European football, with leagues and tournaments cancelled, while the fate of the Wimbledon tennis tournament could be decided next week.

'Wartime level of investment'

The economic damage of the virus -- and the lockdowns -- could also be devastating, with fears of a worldwide recession worse than the financial meltdown more than a decade ago.

But financial markets rose after US leaders agreed a stimulus package worth roughly 10 percent of their economy, an injection Senate Majority Leader Mitch McConnell said represented a "wartime level of investment."

Meanwhile, more than half of all Americans have been urged to stay at home, including in the largest state of California.

The country has at least 62,000 cases and 869 people have died.

New York Governor Andrew Cuomo said the state had 285 deaths and that cases were projected to only peak in 21 days with 30,000 ventilators needed by then.

President Donald Trump has voiced hope the US will be "raring to go" by mid-April, but his optimism appeared to stand almost alone among world leaders.

Anxious Indians raced for supplies after the government ordered the world's second-biggest population not to leave their houses for three weeks from Wednesday.

"To save India, to save its every citizen, you, your family... every street, every neighbourhood is being put under lockdown," Prime Minister Narendra Modi said.

Whether the order will be obeyed in full remained to be seen. Mumbai vegetable trader Rafiq Ansari said his customers were getting angry over shortages and price hikes.

"I don't understand what's going on," the 35-year-old told AFP. "We are going to face major shortages in the days ahead."

Many governments are listening to health experts who warn the only way to slow the epidemic -- and save the lives of the elderly and vulnerable -- is by imposing "social isolation" measures.