A day after shares of U.S. retailer GameStop rose 135% ¡ª a wild upswing spurred by an online army of investors on a mission to challenge the dominance of Wall Street ¡ª Robinhood, the stock-trading app at the center of it all, clamped down.

Almost immediately, GameStop¡¯s shares plunged, falling 75% in 90 minutes.

The limits on trading by Robinhood and other online brokerages, put in place as fears of market instability grew more widespread, set off a furious outcry among small investors. They claimed that the very apps that had democratized trading ¡ª Robinhood in particular ¡ª were now doing the bidding of Wall Street.

Small groups of investors protested outside the New York Stock Exchange and at the Menlo Park, California, headquarters of Robinhood, a company that popularized the notion of commission-free trading. At least one aggrieved trader filed a lawsuit. Politicians on both the left and right weighed in. Before long, a trading strategy involving an obscure stock had morphed into a symbol of class warfare ¡ª pitting young upstarts against established Wall Street investors.

¡°You know the story of Robin Hood, this disenfranchised group of rebels stealing from the elite establishment and gifting that money to the poor,¡± said Travis Shetler, 21, a college student in Meadville, Pennsylvania, who bought shares of GameStop and Nokia in recent days. ¡°The irony of an app like that pandering to the elite is quite obvious.¡±

Robinhood said the additional restrictions on trades of GameStop and some other companies were necessary to ensure it didn¡¯t run afoul of government regulations. The curbs allowed users to only sell shares they owned and repurchase shares they had borrowed and sold, effectively halting the kinds of options bets that drove the rally. It also raised margin requirements for certain securities, meaning investors needed to keep more of their own money in their accounts when placing trades using funds they had borrowed from Robinhood. It later halted all margin trading involving GameStop and the movie theater chain AMC.

Webull, another trading app, said customers could only liquidate positions they already had in GameStop. Another, Interactive Brokers, put its own guardrails around several popular securities.

GameStop shares recovered some losses to end the day down 44%. The shares then rebounded significantly in the after-hours trading session, after Robinhood said it would ¡°allow limited buys of these securities¡± on Friday. Many Robinhood customers used Twitter to post notices from the firm letting them know it had sold off some of their positions, without their permission. The firm said ¡°unprecedented volatility¡± and an effort to help reduce risk had prompted it to sell shares and options contracts for GameStop and AMC.

Users were livid.