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Social media users flooded TD Ameritrade's and Fidelity Investments' Twitter feeds with irate messages on Friday, as customers struggled to access their accounts with the online brokers.

The apparent problems came on the heels of the British referendum to leave the European Union.

Global financial markets plunged as results from the vote showed a 52-48 percent victory for the campaign to leave a bloc Britain joined more than 40 years ago.

A TD Ameritrade sign is seen outside a branch in the financial district in New York February 29, 2016. REUTERS/Brendan McDermid

"@TDAmeritrade Really? Your service is down after BREXIT? Good job TD," tweeted Mo Karney.

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"Just opened my TD Ameritrade app and its a damn massacre!" tweeted Pablo Suarez. "Thanks a lot #BREXIT!"

"Real bad time for @Fidelity to have their website crash," tweeted Fan Audibles.

On Twitter, TD Ameritrade employees tried to reassure customers, saying they were working to resolve any problems and encouraging customers to call a 1-800 number to place trades.

Fidelity, meanwhile, insisted in tweets that their website was operational, and encouraged clients to call in to troubleshoot their problems.

"This morning's heavy volumes across the industry caused some brief messaging and log-in delays," Joe Giannone, a TD Ameritrade spokesman, wrote by email. "Everything is now functioning normally."

Fidelity did not immediately respond to a request for comment.

U.S. stocks plunged on Friday, with the Dow Jones average falling more than 500 points.