Turkey's broadcasting watchdog has launched an inquiry into six opposition TV channels for "insulting the public" with their coverage of Sunday's presidential election runoff, it said on Tuesday, without detailing what the insults were, Reuters reports.

The Radio and Television Supreme Council (RTÜK) said viewers had complained.

President Tayyip Erdogan extended his two-decade rule in the second round of the election on Sunday. His rival, Kemal Kilicdaroglu, said it was "the most unfair election in years" but did not dispute the outcome.

RTÜK penalized four television stations in March for their election coverage. International rights groups and Turkish opposition parties have accused the RTUK of attempting to silence opposition media as a tool of the government.

"The government's censorship device is at work," Tele 1, one of the channels being investigated, said on its website.

Turkey came in at 153 out of 180 in Reporters Without Borders' press freedom index last year, classifying the government's control of the media as high.

"During the election period, RTUK acted like an apparatus of the governing alliance to penalize critical coverage and independent journalism," said Gurkan Ozturan, coordinator of Media Freedom Rapid Response at the European Centre for Press and Media Freedom.

"This kind of treatment of media is unacceptable and creates unfair conditions for competing parties and candidates, also putting a shadow on the results."