Some Fidelity and Charles Schwab clients experienced difficulties accessing their accounts amid the 800-point drop in the markets Monday.

Both brokers told CNBC about an hour after the open that the issues had been resolved.

"Due to higher-than-usual volumes, some clients may have experienced delays in accessing some online features as the market opened but our systems are fine and up and running," Schwab Public Relations told CNBC. Schwab manages about $4 trillion in assets.

"Some clients are experiencing technical issues and we are working as quickly as possible to resolve," Fidelity told CNBC shortly after the open. Privately held Fidelity, the largest online broker, manages about $7.8 trillion in client assets for nearly 30 million clients.

TD Ameritrade, also experienced "some slowness in the reporting of trade confirmations this morning related to heavy trade volumes at the open," the broker told CNBC. TD Ameritade has about $1.4 trillion in client assets.

Fidelity clients took to Twitter to air their concerns about the technical issues. "@Fidelity it would be great on a day like today if your site worked," said one Twitter user.

Tweet

Many photos on Twitter showed that Fidelity's platform was telling its clients their account balance was $0.00. Schwab's app was flashing a "temporarily unavailable" message, according to photos on Twitter.

The Dow Jones Industrial Average fell more than 800 points on Monday as the number of coronavirus cases outside China surged, raising fears of a prolonged global economic slowdown from the spreading deadly virus. The Dow's drop was the third-largest one-day decline by the 30-stock index in the past three years.

Shares of Schwab dipped 5.2% and TD Ameritrade sunk 5.5% amid the market sell-off on Monday.

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— With reporting from CNBC's Jim Forkin.

Correction: Schwab manages about $4 trillion in assets. An earlier version misstated the figure.