By Shashwat Chauhan

March 19 (Reuters) - Most Latin American currencies fell on Wednesday, as risk assets took a hit after Turkish authorities detained President Tayyip Erdogan's main political rival, while focus was on interest rate decisions due later in Brazil and the United States.

Turkish assets were hit hard after authorities detained Istanbul mayor Ekrem Imamoglu, the main rival of President Tayyip Erdogan, on charges of corruption and aiding a terrorist group in what the main opposition party called "a coup against our next president".

The lira TRYTOM=D3 plunged as much as 12.7% earlier in the day to a record low of 42 per dollar, though it pared some losses and was last down 3.6%. Local stocks .XU100 dropped more than 9% while its international bonds also saw heavy selling pressure.

Turkish Finance Minister Mehmet Simsek said that everything necessary is being done to ensure the markets are functioning healthily after the slide, without giving any further details.

The central bank sold a record amount of foreign currency on Wednesday, according to calculations by three bankers.

Analysts wondered how Wednesday's slide would affect monetary policy. In December, the central bank cut rates for the first time after an 18-month tightening effort that had reversed years of unorthodox policy and easy money backed by Erdogan, who has supported the U-turn.

"The impact of political news from Turkey is being observed in price action across EM, as Turkey remained a significant carry position in most EM portfolios," Citi analysts said in a note.

Globally, eyes would be on the Federal Reserve's interest rate decision at 1800 GMT (2:00 p.m. ET), with consensus expecting the central bank to hold its rate steady, though recent uncertainty around U.S. trade policies and fears of an economic slowdown have led to markets pricing in greater easing by the year-end.

"While the rate decision is largely priced in, market attention will shift toward the Fed's new economic projections, and most importantly, to Federal Reserve Chair Jerome Powell's press conference," said Quasar Elizundia, research strategist at Pepperstone.

Brazil's real BRL= firmed 0.1% ahead of a local interest rate decision later in the day, when it is widely expected to hike rates to bring the Selic rate to 14.25%.

Mexico's peso MXN=, which has seen heavy volatility amid U.S. President Donald Trump's tariff threats, was down 0.5% on the day.

Earlier this month, Trump exempted goods from Canada and Mexico under a North American trade pact from his 25% tariffs, with the exception set to expire on April 2.

Chile's peso CLP= held firm at 916.96 per dollar in the lead-up to a local interest rate decision on Friday, with expectations of the central bank holding steady at 5%.

MSCI's index for Latin American currencies .MILA00000CUS was flat on the day, though it remained nearly 10% higher on a year-to-date basis.

Most EM currencies have cashed in on the global weakness of the dollar amid the lingering uncertainty around U.S. trade policies.

Local stocks .MILA00000PUS were up 0.9%, with Argentina's Merval .MERV jumping 2.1%, while heavyweight Brazil .BVSP added 0.9%.

Latin American stocks have had a upbeat year so far, rising close to 16% YTD, in contrast to the U.S. S&P 500 .SPX, which was last down almost 4%.

Elsewhere in emerging markets, Ukraine's international bonds and the Russian rouble RUB= fell after Russian President Vladimir Putin agreed on Tuesday to stop attacking Ukrainian energy facilities temporarily but declined to endorse a full 30-day ceasefire.

Ukrainian President Volodymyr Zelenskiy said he would speak to President Donald Trump on Wednesday.

Key Latin American stock indexes and currencies:

(Reporting by Shashwat Chauhan in Bengaluru; Editing by Alexandra Hudson)

((Shashwat.Chauhan@thomsonreuters.com;))