A day before the presidential election in Mauritania, the police arrested the leading opposition candidate on Thursday only to release him without charge. The incident has raised concerns over transparency at Friday's election.

The candidate, Mohamed Khouna Ould Haidallah was held aloft on the balcony of his campaign center while women ululated and men chanted, "Haidallah, president! Haidallah, president!" from the terrace below.

Mr. Ould Haidallah was detained at the national police headquarters under the jurisdiction the political police, and questioned he said about plans to set up an illegitimate government and stage a coup.

The opposition leader said that he had refused to answer the questions that the police had posed him. He rejected the affair as an attempt by the government to discredit his candidacy.

Mr. Ould Haidallah believed it was the intervention of unnamed foreign governments that asserted pressure on the government of Mauritania to secure his release.

He called on the electorate to continue to turn out and observe the opportunity to vote Friday. He said that he would stand by the results of the vote providing that they were carried out in the proper manner though he admitted that his own arrest was not in keeping with the spirit of a transparent election.

According to Mr. Ould Haidallah's son-in-law Ould Elvadhel, a dozen armed policemen arrested Mr. Ould Haidallah by force Thursday afternoon.

Mr. Ould Elvadhel said that police entered the house violently and swiftly, and grabbed Mr. Ould Haidallah by the wrist to take him away.

Mr. Ould Haidallah is a former head of state who came to power through a coup d'état, and was later deposed by the current president Maaouiya Ould Taya, in another coup.

Police searched the family home on Tuesday and say they confiscated weapons that could have been used to make trouble after the election. But Mr. Ould Haidallah's spokesman says they were just two old hunting rifles.

Both of Mr. Ould Haidallah's sons were arrested Wednesday.

His supporters say the government is trying to slur Mr. Ould Haidallah's name ahead of Friday's presidential voting.