* S&P cuts U.S. outlook, China reserve requirements up
* Brazil's OGX plummets 17.3 pct on crude reserves doubts
* Brazil's Bovespa off 1.9 pct, Mexico's IPC off 1.77 pct (Adds comments and closing prices)
MEXICO CITY/SAO PAULO, April 18 (Reuters) - Latin American stocks slumped on Monday as tighter Chinese lending as well as worries about debt levels in the United States and the euro zone pushed investors to dump riskier assets like equities.
The MSCI Latin American stocks index <.MILA00000PUS> fell 2.69 percent in its biggest one-day drop in almost six months. The gauge closed at a three-week low and regional indexes sank below technical supports that could bode for more losses.
"There is the feeling that growth is going to be a bit more negative compared to the optimism at the beginning of the year," said Rodolfo Navarrete, head of analysis at brokerage Vector in Mexico City.
"If strong earnings reports are able to offset this, the change in tendency may not be that deep," he added.
Equities sank around the world on a slew of worrying news.
Standard & Poor's revised downward its long-term credit outlook for the United States, a major Latin American trading partner. For more see [ID:nN18195555].
In Europe, pressure on financially weak euro zone members rose amid growing speculation Greece will restructure its debt and concerns Portugal's request for a bailout could face political resistance. [ID:nLDE73H0IC]
In Asia, China raised banks' required reserves on Sunday for the fourth time this year, stoking concerns that slower growth in China could dampen global demand for Latin America's key commodities, such as Chile's copper and Brazil's iron ore. [ID:nL3E7FH05Y]
"There is nothing, not one clear sign that anything is getting better," said Daniel Marques, an equity analyst with Agora. "Today it's not just one thing weighing, it's a combination of factors."
Brazil's benchmark Bovespa stock index <.BVSP> tumbled 1.9 percent to 65,415.49 to close at its lowest since Feb. 11. Volume spiked to its highest since Feb. 23, according to Reuters data.
The index fell through support at 65,500, which could suggest further weakness ahead, analysts said.
Shares of oil company start-up OGX
State-controlled energy company Petrobras
Mexico's IPC index <.MXX> declined 1.77 percent in its biggest drop since August last year and the gauge closed at its lowest in more than three weeks.
The index sank below its 50-day simple moving average and fell through a key support seen at 36,600, suggesting more losses could be on the way.
America Movil
Chile's IPSA index <.IPSA> dropped 0.56 percent as
fertilizer, lithium and iodine producer Soquimich (SQM)