HARARE, Zimbabwe — The leader of Zimbabwe’s opposition was detained by police for about nine hours yesterday amid ominous signs the government is tightening its grip on the country less than four weeks before a presidential runoff election.

Morgan Tsvangirai, who returned 12 days ago to face President Robert Mugabe in the June 27 ballot, was taken with about 14 others from his Movement for Democratic Change to a police station in Lupane, north of Bulawayo, his spokesman said.

Spokesman George Sibotshiwe was quoted in a party statement as saying Mr. Tsvangirai was released after being charged with a public order offense that he described as “a spurious charge of attracting a large number of people.”

Mr. Tsvangirai had to sign an official police caution before he was let go and one of his security vehicles was seized, the statement said.

“This is yet another shameless and desperate act of this illegitimate regime to try and subvert the will of the people of Zimbabwe,” the Movement for Democratic Change said.

Mr. Tsvangirai’s detention had been condemned by America, Britain, Germany and the human rights group Amnesty International.

Among those with Mr. Tsvangirai were the party’s vice president, Thokozane Kupe, and its chairman, Lovemore Moyo. They were stopped by police at a roadblock while campaigning in towns north of Bulawayo, Zimbabwe’s second-largest city, according to Mr. Tsvangirai’s spokesman.

Mr. Tsvangirai, 56, insists he won the first round of the election outright and says official results released May 2 showing a runoff was needed were fraudulent.

“Mugabe is determined to turn the whole country into a war zone in order to subvert the will of the people and steal the June 27 election by any means possible,” Mr. Tsvangirai said in Bulawayo earlier yesterday.