ATHENS, Greece (AP) — Greece signed an agreement Friday to sell a 67 percent stake in operating its biggest port to China's Cosco group, a significant step in the debt-crippled country's ambitious privatization drive.

Dockworkers protesting in central Athens against the sale, which Greece's left-led government had initially opposed, clashed with riot police who responded with tear gas. No arrests or injuries were reported.

Cosco, owned by the Chinese government, was the sole bidder for the stake in Piraeus Port Authority, a listed company that has a concession to operate the port until 2052.

Greece will retain a 7 percent stake in PPA and private investors will hold the remainder. Cosco's 368.5 million euro ($420 million) offer was approved in January. The deal still needs to be approved by Greece's competition commission and Parliament.

Cosco already has a major presence in Greece following a 2008 concession agreement to manage container terminals at Piraeus.

Piraeus, the port of Athens, is the main gateway to the Greek islands and in 2014 handled 16.8 million passengers.

The Greek government is trying to establish closer ties with China. On Friday, it said Prime Minister Alexis Tsipras, who attended the PPA signing ceremony, has accepted an invitation to visit China in June.