* Poll key to return to civilian rule
* Stability seen opening door to investment
* Tandja party at centre of broad alliance
NIAMEY, Feb 3 (Reuters) - A party ally of deposed Nigerien leader Mamadou Tandja will head into a decisive presidential run-off vote next month with the backing of a broad coalition, results of a first-round poll showed on Thursday.
The election is the culmination of efforts to return the uranium-exporting Sahel country to civilian rule after soldiers a year ago ousted Tandja for outstaying his term in office.
Ex-premier Seyni Oumarou of Tandja’s MNSD party came second in the first round with 39 percent of Sunday’s vote against 43 percent for Mahamadou Issoufou of the opposition PNDS, according to results read out on state television.
But Oumarou will be favourite to win as the MNSD is part of an alliance of around 20 parties which agreed a week before the Jan. 31 first-round to seek a coalition government together.
The last-minute coalition pact drew criticism from opponents of the MNSD, who argue it cannot be trusted with power after it helped Tandja force through law changes expanding his powers and scrapping term limits. Tandja is being held on fraud charges.
Junta leader Salou Djibo, who came to power after a February 2010 coup against Tandja, has been praised for his promise to leave power by April this year.
A poor desert nation, Niger’s uranium riches have drawn billions of dollars worth of investments, mainly from French nuclear giant Areva CEPFi.PA.
In an attack highlighting the growing menace of al Qaeda in the region, it claimed the kidnapping of two Frenchmen in the Nigerien capital Niamey last month. The pair were later found dead in the desert after a failed French-Nigerien rescue bid. (Writing by Mark John; editing by Maria Golovnina)