* Investors pocket gains after Fed announcement last week
* MSCI Latin America stock index off 0.3 pct
* Brazilian shares gain as much as 1 pct in choppy session
* Brazil Bovespa up 0.71 pct, Mexico IPC down 0.36 pct
By Danielle Assalve and Silvio Cascione
SAO PAULO, Sept 17 (Reuters) - Latin American shares slipped in a choppy session on Monday as investors pocketed gains after a key regional stock index recorded its strongest weekly gain since mid-January.
The MSCI Latin American stock index fell 0.27 percent to 3,795.82, after rising 4.8 percent last week following the announcement of a third round of monetary stimulus in the United States.
A technical indicator known as the relative strength index had crossed into "overbought" territory on Friday, indicating stocks were due to fall in following sessions.
Mexico's IPC index fell after four straight gaining sessions, falling 0.36 percent to 40,546.17 in thin volume.
"The only thing that moved the market last week was the hope of a Fed announcement, but now there is no gas in the market," said Mauricio Cervantes, trader at Multivalores brokerage in Mexico City.
Brazil's benchmark Bovespa stock index advanced 0.71 percent to 62,546.41, near its highest level in more than four months.
Earlier, the Bovespa index rose as much as 0.9 percent to an intraday high at 62,691.86 after a weak opening, though, as the expiration of futures and options fueled market volatility.
"It would be natural to see a pullback after the Bovespa index surpassed 60,000 points, but it seems that this correction will be mild, with the index trading sideways," said Leandro Martins, chief analyst at Walpires Corretora, in Sao Paulo.
The benchmark index for Brazilian stocks has gained nearly 10 percent in September so far.
Last week, the U.S. Federal Reserve launched an aggressive monetary stimulus program on Thursday. The Fed pledged to buy $40 billion of mortgage-backed securities per month and indicated it would probably leave interest rates at current rock-bottom lows until at least mid-2015.
"An avalanche of good news caught the market oversold last week. Now all those things need to become reality. Investors will wait as they consolidate recent gains," said Raffi Dokuzian, managing director at CGD Securities, in Sao Paulo.
Shares of state-controlled oil company Petrobras climbed 1 percent, while shares of mining giant Vale added 1.03 percent. In contrast, preferred shares of steelmaker Usiminas dropped 3.53 percent.
In Mexico, retailer Wal-Mart de Mexico was almost unchanged, down 0.04 of a peso at 36.64 pesos, a dip of less than 0.01 percent. Shares of cement manufacturer Cemex also edged lower, off 0.01 of peso at 10.590 pesos.
Chile's IPSA index gained 0.79 percent, rising for the seventh session in eight as retailer Falabella climbed 1.6 percent.
Latin American prices at 1633 GMT Stock indexes daily % YTD %
Latest change change MSCI LatAm 3,795.82 -0.27 5.66
Brazil Bovespa 62,546.41 0.71 10.21
Mexico IPC 40,546.17 -0.36 9.36
Chile IPSA 4,218.00 0.79 0.97
Chile IGPA 20,486.10 0.71 1.77
Argentina MerVal 2,492.48 -2.01 1.21
Colombia IGBC 14,480.10 -0.11 14.33
Peru IGRA 21,891.42 -0.28 12.42
Venezuela IBC 295,729.59 -0.02 152.68
(Reporting by Silvio Cascione and Danielle Assalve in São Paulo and Gabriel Stargardter in Mexico City; Editing by Jan Paschal)