MINSK, November 29 (RIA Novosti) - Belarusian opposition leader Alexander Milinkevich was arrested Wednesday at Minsk airport after returning from a trip to the Latvian capital, on suspicion of forging his domestic passport, the leader's press secretary said.

Riga was the venue of the North Atlantic Treaty Organisation's first summit on former Soviet territory, on Tuesday and Wednesday.

Milinkevich, who has been arrested several times over his role in mass demonstrations that followed the March elections in Belarus, visited Riga on the invitation of the Latvian president and met with U.S. President George Bush and several White House officials who were attending the NATO summit.

"I would not want to make pessimistic assumptions, but forgery is considered a criminal offense, and the consequences may be serious," Pavel Mozheiko said, adding that Milinkevich, who previously faced only administrative charges, could be kept in custody for a considerable time.

Milinkevich contested Belarus' presidential elections in March as the main opposition candidate, but finished a distant second, with 6% of the vote, behind incumbent President Alexander Lukashenko, who officially won 83% of the vote. The opposition and international observers said the poll was rigged.

In April, a court sentenced him to 15 days' detention for his part in an unauthorized 3,000-strong rally organized by opposition groups.

The rally was called by the opposition to coincide with the 20th anniversary of the Chernobyl disaster, the world's worst nuclear accident, which affected much of the country.