Officials say bidders include Telecom Italia and a joint venture linking Emirates Telecommunications Corporation with Cetel Calik Enerji and Dubai Islamic Bank. Another bidder is Saudi Oger Ltd, which includes Saudi Telecom.
A subsidiary of leading Turkish mobile phone operator Turkcell, Turktell Bilisim, will lead a 14-member consortium. Turkish business group Oyak and a group called Turkish Privatisation Investors are also bidding.
Several companies including Belgium's Belgacom SA and Spain's Telefonica SA have withdrawn from the planned sale.
The sale of Turk Telekom's controlling stake is a key element of Turkey's IMF-backed privatisation programme which has yet to complete a single major sale due to legal challenges and unsatisfactory bid prices.
No realistic value estimate has yet emerged for Telekom, which has 19 million subscribers, and according to sector analysts is the world's 13th-largest telecoms group.
Telekom technically lost its monopoly status last year after the government opened the market to competition, but still controls the market with no rivals yet in the landline sector.