Also lifting investor sentiment: trading platforms restricted transactions in several heavily shorted stocks, easing fears about a ripple effect to the broader market. And more measures could follow.

Invesco US Value Equities chief investment officer Kevin Holt:

"It seems like regulators are stepping in, trying to get some control so hopefully, things calm down over the next week or so."

Shares in red-hot AMC fell by more than half and GameStop 44% after some online brokerages curbed trading of those stocks. That capped a spectacular rise in recent days sparked by a social media-driven frenzy.

American Airlines, a heavily shorted stock, initially joined the fray and shot up more than 30% at the market open but settled for a 9% rise as Robinhood restricted trading. The carrier reported earnings that beat analysts expectations, but investors said those were not enough to justify the stock's big advance.

The Dow and S&P 500 ended the day with a gain of almost 1%. The Nasdaq added a half percent.