SHANGHAI  After three decades of spectacular growth, China passed Japan in the second quarter to become the world’s second-largest economy behind the United States, according to government figures released early Monday.

The milestone, though anticipated for some time, is the most striking evidence yet that China’s ascendance is for real and that the rest of the world will have to reckon with a new economic superpower.

The recognition came early Monday, when Tokyo said that Japan’s economy was valued at about $1.28 trillion in the second quarter, slightly below China’s $1.33 trillion. Japan’s economy grew 0.4 percent in the quarter, Tokyo said, substantially less than forecast. That weakness suggests that China’s economy will race past Japan’s for the full year.

Experts say unseating Japan  and in recent years passing Germany, France and Great Britain  underscores China’s growing clout and bolsters forecasts that China will pass the United States as the world’s biggest economy as early as 2030. America’s gross domestic product was about $14 trillion in 2009.