ATHENS, Greece -- Greece's left-wing government launched an auction Tuesday for four private national television licenses, reducing the number from seven after a heated public debate on corruption in the financially troubled country.

In a heavily policed government building, eight bidders are taking part in the process that would see eliminated TV stations close by the end of the year. Private Mega Television was excluded last month because of financial difficulties.

Dozens of TV station executives and advisers, most wheeling suitcases into the building, won't be allowed to leave or communicate with rivals or the outside world until the bidding process is completed.

Bids were to start at 3 million euros ($3.35 million) and increase by amounts of 500,000 euros ($560,000).

"In order to safeguard the interest of the state, the flow of information between the bidders must be cut," Nikos Pappas, a minister of state whose office supervised the auction process, said on state TV.

"What is paramount is that the rules of the bidding process are clear and that the state receives the maximum amount (of money)."

The government argues the process to grant 10-year licenses is long overdue. It replaces a system based on short-term renewals -- starting after private channels first appeared 27 years ago -- that it argues had fostered corrupt arrangements between businesses and political parties.

The opposition conservatives accuse Prime Minister Alexis Tsipras' government of market interference and attempting to exert state influence over private news media.

"The restriction of broadcast licenses ... will distort competition and does not ensure that the public will be informed objectively objective information to citizens," the centre-right New Democracy party said.

"Concentrating power in the news media, restricting pluralism and the range of opinion is essentially taking aim at democracy itself."

The party said it wouldn't recognize the results of the auction, arguing that legal challenges to the bidding method are still pending and that an independent regulator had been left out of the process.

Television license rules are a highly contentious issue in Greek politics. The previous conservative-led government shut down public broadcaster ERT in 2013, and later replaced it with a scaled-down channel that it argued would operate more efficiently.

ERT was reopened by the Tsipras government last year, and laid-off workers were reinstated.

The final results of the auction are expected late Wednesday.