ULAANBAATAR (Reuters) - The main opposition Mongolian People's Party (MPP) appears to have swept back to power in parliamentary elections, Mongolian media said early on Thursday, after campaigning dominated by concern over slowing economic growth.

The transformation of the former Soviet bloc state since a peaceful revolution in 1990 has been a big draw for foreign investors eyeing its rich mineral resources, unleashing a boom from 2010 to 2012.

But an abrupt economic slowdown since 2012 has stirred controversy over the role of global mining firms such as Rio Tinto, which last month finally approved a $5.3-billion extension plan for the Oyu Tolgoi copper mine.

The MPP, which last ruled from 2008 to 2012, is likely to have won most seats in the 76-member parliament, taking back power from the Democratic Party, leading Mongolian news websites news.mn and Ikon reported.

The government was expected to formally announce the final results of Wednesday's vote later on Thursday morning.

Stability during the MPP's rule helped lure investors to Mongolia, a vast country with just three million people that was nicknamed "Mine-golia" in the boom years.

But it has since struggled to adapt to giant neighbor China's reduced appetite for coal and copper, which has hit commodities prices.

The IMF forecasts economic growth of 0.4 percent this year, compared with 17.5 percent in 2011, the year before the Democratic Party took power.

Since 2012, Mongolia has borrowed billions of dollars in sovereign debt. In March, rating agency Moody's gave it a negative outlook, citing the rising debt burden, a projected widening of budgetary imbalances and mining revenue shortfalls.

The MPP, which has held power most years since 1990, has criticized the Democrats' economic management and the borrowing spree, promising to reassess spending and tighten fiscal management.

(Reporting by Terrence Edwards; Writing by Ben Blanchard; Editing by Andrew Roche)