A day after shares of U.S. retailer GameStop rose 135% — a wild upswing spurred by an online army of investors on a mission to challenge the dominance of Wall Street — 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 — Robinhood in particular — were now doing the bidding of Wall Street.