(Adds response from Texas Pacific)

May 22 (Reuters) - An investor group that holds about 25% in Texas Pacific Land Trust installed its preferred trustee in a shareholder meeting on Wednesday that the company, one of the biggest landowners in Texas, rejected as invalid.

On Tuesday, Texas Pacific postponed the special meeting where shareholders were due to fill a vacant trustee spot after filing a lawsuit against the investor group's trustee candidate, Eric Oliver, a source familiar with the matter told Reuters.

After the meeting was held anyway, the company accused the investor group of misleading investors.

"Shareholder meetings may only be called after certain requirements are met, including providing notice and following other procedures," Texas Pacific said in a statement.

"Mr. Oliver's purported 'meeting' was conducted in a secretly-booked conference room with no notice whatsoever provided to the vast majority of the shareholders," it said.

Eric Oliver is president of hedge fund SoftVest LP, one of the investor group along with Horizon Kinetics LLC and ART-FGT Family Partners.

The group wants to convert Texas Pacific, which has been operating since 1888, into a Delaware corporation subject to modern governance principles, to establish an experienced team around its water business and be more transparent. It also wants to sell off the firm's portfolio.

The proxy contest illustrates how even strongly performing companies can run afoul of activist investors at a time both experienced corporate agitators and newcomers are pushing corporations to perform better to help boost their own returns.

In this case, the activists hope to cash in on excitement around the Permian basin, which covers Western Texas and parts of Eastern New Mexico and which has been the center of the U.S. shale revolution.

Texas Pacific wants shareholders to elect an independent trustee, and alleges Oliver is misleading shareholders by "making materially misleading statements and failing to disclose material information."

"The Trust will announce a new date for the special meeting once the claims and issues raised in litigation are resolved", the company said in a statement. (Reporting by Debroop Roy and Kanishka Singh in Bengaluru Editing by Shailesh Kuber and Sonya Hepinstall)