Trump released guidelines Monday instructing Americans to avoid social gatherings of more than 10 people for the next 15 days to slow the spread of the pandemic. The guidelines also advise Americans to avoid eating and drinking at bars, restaurants and food courts; going on shopping trips and making social visits; and visiting nursing homes and retirement or long-term care facilities.

"The best thing I can do for the stock market is we have to get through this crisis, that’s what I can do," Trump said at a White House briefing with reporters on the coronavirus response Monday afternoon.

"Once this virus is gone I think you're going to have a stock market like nobody's ever seen before," he said.

As he spoke, the markets continued to rack up losses. The president, who had initially downplayed the seriousness of the virus, today seemed to acknowledge that its spread had gotten worse than expected and that it could last months.

"It's bad, it's bad," he said.

The White House is aiming to spend at least $800 billion in the coming weeks to bolster the economy, as retailers, restaurants, sporting events and other businesses shut down and Americans slow their spending while staying home to guard against the pandemic.

The day started off ominously, with the stock exchanges being forced to shut down trading within minutes of the market opening, as coronavirus fears prompted a massive sell-off.

The S&P 500 declined 8 percent after trading started at 9:30 a.m., resulting in a mandatory 15-minute break under Securities and Exchange Commission rules. The pause was the third in two weeks and only the fourth in history.

Stocks continued edging down in the afternoon in another bruising day. All day the possibility of another market-wide trading halt loomed as all three major indices — the S&P 500, the Dow Jones Industrial Average and the Nasdaq composite were down about 9 percent, before falling even further at the close.

SEC Chairman Jay Clayton tried to reassure investors, saying in an interview on CNBC on Monday morning that "markets should continue to function through times like this."

"Things have been functioning well in our markets over the past two weeks of a very volatile time," Clayton said.

Monday's trading results demonstrated that markets are bracing for further drops as the coronavirus shows no sign of abating and the U.S. government's response has come under criticism.