MOSCOW — Kazakhstan’s currency, the tenge, plunged by about a quarter on Thursday after the central bank stopped managing the exchange rate, prompting fears that a wider emerging market currency rout could be coming.

The surprise move in Kazakhstan was in response to falling prices of oil, the country’s main export, and earlier devaluations by its two main trading partners, Russia and China.

Also on Thursday, Vietnam devalued the dong, and currencies as far afield as the Brazilian real and the South African rand started to skid.

The next to drop, financial analysts say, will most likely be the currencies of Kazakhstan’s neighbors in Central Asia, a poor region with a population of about 50 million people: the Kyrgyz som, the Turkmen manat and the Tajik somoni.