TD Ameritrade, owned by Charles Schwab (SCHW), said in a statement that "in the interest of mitigating risk for our company and clients, we have put in place several restrictions on some transactions in Gamestop and AMC and other securities."
"We made these decisions out of an abundance of caution amid unprecedented market conditions and other factors," TD Ameritrade added.
GameStop more than doubled Wednesday and has soared nearly 720% in just the past five days. AMC skyrocketed more than 200% Wednesday.
Both stocks have been propelled by individual traders on Reddit's WSB board as well as customers of popular brokerage firm Robinhood. Each stock has been targeted by short sellers who believe the recent price surges are overdone.
But since short sellers have to borrow stock and sell it -- and eventually have to buy back the shares and return them -- the bullish comments and buy-by-the-day traders are causing big losses for hedge funds betting against GameStop and AMC. That has driven the stocks even higher, known as a short squeeze.