(CNN) -- Zimbabwe presidential contender Morgan Tsvangirai was released Wednesday after spending nearly 10 hours in police custody, a Tsvangirai spokesman said.
Tsvangirai, head of the opposition Movement for Democratic Change party, and several other MDC leaders were detained Wednesday because they were going to attend an unauthorized meeting in southern Zimbabwe, MDC party spokesman Nelson Chamisa said.
No charges were filed, but authorities kept custody of the vehicles that the group was traveling in, said Chamisa, who communicated with journalists via text message Wednesday after going into hiding.
Tsvangirai was picked up by police around 2 p.m. Wednesday, less than a month before the run-off election against President Robert Mugabe.
MDC has accused Mugabe and his supporters of orchestrating a reign of violence ahead of the June 27 run-off, using beatings and dozens of arrests in a attempt to sway the results.
Mugabe, who is in Rome, Italy, attending a United Nations World Food Summit, has disputed these claims, saying that MDC party members have concocted stories to get international support.
But on Sunday, Arthur Mutambara, the head of an MDC faction, was arrested on charges of criticizing Mugabe's government.
At least 30 aid agencies that provide food, health care and other services to some of the poorest people in Zimbabwe were also told to suspend their work in the country after the government accused them of helping Tsvangirai's party.
The fallout from Zimbabwe's stalled election has brought a chorus of international criticism, including from the United States.
"The continued use of government-sponsored violence in Zimbabwe, including unwarranted arrests and intimidation of opposition figures, to prevent the Movement for Democratic Change from campaigning freely ahead of the June 27 presidential runoff election is deplorable," U.S. President George Bush said Monday.
Not long after news broke Wednesday of Tsvangirai's detention, the international community began calling for his release.
"A tragedy is unfolding in Zimbabwe," U.S. State Department spokesman Sean McCormack said. "There has been a dramatic retrenchment in political freedoms in the country. We are deeply concerned about whether or not the proper conditions for a free and fair election can be brought about."
The first election was March 29. After weeks of delay, the Zimbabwe Electoral Commission ruled neither candidate won the required majority of votes, and scheduled a runoff election for June 27.
Tsvangirai, who claims he won the first election, wants international observers to monitor the second round of voting, but Mugabe's government has declined to allow it.
This is not the first time Tsvangirai has faced alleged intimidation attempts. He fled the country to South Africa in March because, he said, he was being targeted for assassination by the government.
A Zimbabwe Cabinet member denied that claim to CNN at the time.
Last March, Tsvangirai was severely beaten in a police crackdown. The pictures of his battered and swollen face were circulated around the world and became an enduring image of political struggle in the country.