The S&P/TSX Composite had its biggest single-day decline since 1940, plunging 12.34 per cent on Thursday, in a market rout that has gripped global markets amid mounting concerns about the COVID-19 pandemic.

Every TSX subgroup closed in negative territory, led by energy, health care and financials.

The Canadian index also erased four years of gains to trade at levels not seen since 2016, shedding 1,761.64 points during the session to close at 12,508.45 points.

There was also a bloodletting on U.S. markets as investors grew increasingly concerned over the potential economic impact of COVID-19. The Dow Jones Industrial Average closed about 10 per cent lower in its steepest decline since Black Monday in 1987, while the S&P 500 plunged into a bear market, ending the 11-year-long bull run.

U.S. indices bounced briefly midway through the trading day after the U.S. Federal Reserve announced it would expand its purchases of U.S. government securities in order to “address temporary disruptions” in the financing markets.

The intense selling pressure also led to a market-wide Level 1 trading halt shortly after the opening bells on both sides of the border.

Market trading halts have three tiers – seven per cent, 13 per cent and 20 per cent – with the latter resulting in trading being stopped for the remainder of the session.

“The market is obviously having a panic attack,” Tony Dwyer, chief market strategist at Canaccord Genuity, told BNN Bloomberg in an interview.

“It’s been having a panic attack really for the last couple of weeks and historically, when you get a panic attack, we try to think our way through it, we try to put fundamentals to it and frankly, nobody knows what the ultimate impact of the coronavirus is going to be on the global growth rate.”

On Wednesday, the TSX and Dow officially closed in a bear market, falling 20 per cent from their recent February highs.

“There were too many people that needed to get out,” Dwyer added. “Obviously you’re in an emotional marketplace that’s in the panic phase. The structure of the market is different so there’s not a lot to stop it when it’s in one of these ‘whooshes.’”

"Unfortunately, as ugly and as painful as it is, it has to play itself out. You can’t make it stop, you just play with it and look at what you have. As I said before, you talk to your advisor and evaluate your own plan and decide what’s best for your own financial situation.”

That sentiment was echoed by Norman Levine, managing director at Portfolio Management, who told BNN Bloomberg on Thursday that the meltdown will eventually end, but it will take a long time to right itself.

“I’m not being glib here but I want people to understand the markets can’t continue to decline the way they currently are,” he said. “If you had another 10 days like this in Toronto, you’d be below zero.”

“If you own bad companies, you’re going to eventually get what’s coming to you in good markets or bad markets, but if you own quality companies and stick with them and don’t panic … even during bad times but the stocks will recover, the dividends will continue to grow, and you’ll be whole again. It might take some time but it happens,” he added.