Zimbabwe's police have launched a new crackdown on the opposition as President Robert Mugabe arrived for a regional summit in Tanzania.
Morgan Tsvangirai was beaten in custody earlier this month
The Movement for Democratic Change said its leader Morgan Tsvangirai was among 20 people detained in a police raid.
The police said 10 people were arrested for suspected petrol bomb attacks but Mr Tsvangirai was not among them.
Mr Mugabe is attending a meeting of a key southern African grouping that is discussing Zimbabwe's situation.
The BBC's Peter Greste in Tanzania's capital, Dar es Salaam, says that in private the gathered leaders of the Southern African Development Community (Sadc) will give Mr Mugabe a frosty reception.
It follows the beating of opposition politicians, including Mr Tsvangirai, in police custody this month.
As Mr Mugabe arrived, the US State Department urged the nations at the summit to call him to account for his "misrule".
In the latest raid, the opposition headquarters in Zimbabwe's capital, Harare, were cordoned off before officers went in to make the arrests.
Mr Mugabe may face some tough talking
Mr Tsvangirai had been planning to hold a news conference about the arrest and assault of scores of opposition activists after police broke up a meeting earlier this month.
Germany, which holds the European Union presidency, said it was "deeply concerned" at the latest arrests.
But Zimbabwe's Information Minister Sikhanyiso Ndlovu told the BBC's Focus on Africa programme the arrests were a police matter.
He added: "You [the West] are too much concerned with your Tsvangirai because he is your puppet and you make him an international hero."
Police spokesman Wayne Bvudzijena said: "I would like to put it on record that Tsvangirai was never arrested." It is unclear if Mr Tsvangirai is in custody.
The BBC's Peter Biles says observers believe Mr Mugabe wanted to use the arrests to show he was still in charge and could contain the opposition while he was attending the Tanzania summit.
In Dar es Salaam, Mr Mugabe is expected to blame tensions in his country on an opposition campaign of violence.
The government has consistently accused the MDC of using violence and attacking the police.
Our correspondent in Tanzania, Peter Greste, says that Mr Mugabe built up strong regional support for standing up to former colonial masters but that is now waning amid the brutal suppression of opposition protests.
The leaders at the two-day summit, starting on Wednesday, are expected to tell Mr Mugabe, who has governed Zimbabwe since its independence in 1980, that he should stand down when his term in office ends next year.
US State Department spokesman Tom Casey said the Sadc should call Mr Mugabe to account for his misrule "not only over the last few weeks but over the last few years".
Our correspondent says Mr Mugabe has so far seemed immune from verbal attacks from the West but may not be so resistant to criticism from his own contemporaries.
The MDC's Tendai Biti told the BBC that Mr Mugabe appeared defiant. "Mugabe is telling them, 'I've got the title deeds to Zimbabwe, you can go to hell'. He's saying, 'Stuff diplomacy'."
But Mr Ndlovu said the summit should go ahead without Western interference.
"The Western countries say, 'We are concerned - there is a crisis in Zimbabwe'. There is no crisis whatsoever in Zimbabwe," he said.
Zimbabweans are grappling with the world's highest inflation - 1,700% a year - while unemployment and poverty are widespread.