GameStop shares sank early today as trading platforms including Robinhood and Interactive Brokers restricted trading in the video game retailer, along with AMC Entertainment and other stocks that soared this week in a social media-driven trading frenzy that shook stock markets.

GameStop, the video game chain whose 1,700% rally has been at the heart of the frenzy in the past week, lost half its value in early trading.

The stock had rallied initially, then retreated, while American Airlines joined the growing list of stocks making stellar gains as small-time traders and major Wall Street institutions broadened their battle.

On Reddit thread WallStreetBets, where calls to buy stocks have helped drive the extraordinary moves, some of its more than 4 million members reported trading platform Robinhood was now preventing investors from buying GameStop and other volatile stocks.

Robinhood cited "recent volatility" in a statement on its website.

It said restricted stocks also included BlackBerry, Koss and Express.

Interactive Brokers, another online trading platform, also said it was restricting trading in those stocks.

"We do not believe this situation will subside until the exchanges and regulators halt or put certain symbols into liquidation only," Interactive Brokers said.

On Twitter, many observers decried the decision to remove certain stocks, arguing the retail trading platforms were trying to protect Wall Street interests at the expense of Main Street.

"Robin Hood: a parable about stealing from the rich to give to the poor. Robinhood: an app about protecting the rich from being short squeezed by the poor," Tweeted Jake Chervinsky, a lawyer for fintech company Compound.

"GameStop 2.0! I do think that some of it is related to short-sellers and those that are looking at some of these short-term opportunities to push stocks around," said Paul Nolte, portfolio manager at Kingsview Asset Management in Chicago.

The 'Reddit crowd,' with its use of call options and coordinated buying, has had a direct impact on the overall stock market.

A basket of stocks traded mostly by hedge funds has fallen 2.5% so far this year, while a basket tracking retail favorites jumped 13.5%, data from Goldman Sachs showed.

GameStop briefly became the biggest stock on the Russell 2000 index of small caps, according to Zerohedge.

The dramatic jumps in the stock price of companies including GameStop, BlackBerry Ltd and AMC drew more calls for regulatory scrutiny.