Wall Street’s superstars are tumbling Monday as a competitor from China threatens to upend the artificial-intelligence frenzy they've been feasting on. The S&P 500 was down 1.7% in afternoon trading and heading for its worst day in more than a month.

The surprise success of a Chinese AI startup is hammering U.S. tech stocks. That includes Nvidia, the California chip behemoth that has added nearly $3 trillion in market capitalization over the past three years.

The S&P 500 fell 1.9% Monday. Big Tech stocks took some of the heaviest losses, with Nvidia down 17.6%, and they dragged the Nasdaq composite down 3.3%.

Wall Street is tumbling on fears the big U.S. companies that have feasted on the artificial-intelligence frenzy are under threat from a competitor in China.

Wall Street is tumbling on fears the big U.S. companies that have feasted on the artificial-intelligence frenzy are under threat from a competitor in China. The S&P 500 fell 1.9%