* Controversially ousted CEO tries to enter office
* Police stopped the clash, 20 people detained
* TPG, VTB, in dispute with largest shareholder Meyer
* Both sides have made own appointments for CEO
ST PETERSBURG, RUSSIA, Sept 14 (Reuters) - A bitter shareholder conflict at Russian hypermarket chain Lenta sparked a fistfight at the firm's headquarters on Tuesday, leading to 20 people being detained by police.
The investor row erupted in August as rival shareholder groups disputed who should be chief executive of the company, recently touted as a potential acquisition target for the world's biggest retailer Wal-Mart.
Lenta's former CEO Jan Dunning, surrounded by armed bodyguards, tried to enter the company's offices on Tuesday but was stopped by Lenta's security guards, witnesses told Reuters.
"I saw advancing guards throw people down from steps and push them against glass doors ... There was blood," Sergei Yushenko, Dunning's replacement as CEO, told Reuters.
Police confirmed they were informed about a fight at Lenta and had detained 20 of the most active participants.
Dunning was ousted by U.S. businessman August Meyer, owner of a 41 percent stake in the firm, in favour of Yushenko, but his departure has not been recognised by fellow investors private equity group TPG, Russian bank VTB and the European Bank for Reconstruction and Development (EBRD).
"This is a criminal offence," Meyer told Reuters, adding it was impossible to work in such conditions.
Lenta has 37 stores and annual turnover of around $1.8 billion. The ultimate cause of the dispute is a perceived failure to expand the group at the pace demanded by Meyer.
Shareholder disputes are common in Russia and are seen as a major barrier to foreign investors. (Reporting by Denis Pinchuk; Writing by Dmitry Sergeyev and John Bowker; Editing by David Holmes)