(BRUSSELS) - The EU presidency called on Zimbabwean authorities to immediately release opposition leader Morgan Tsvangirai after his arrest Wednesday ahead of a presidential election run-off against Robert Mugabe.

"We are deeply concerned on the report of the arrest of Tsvangirai," an EU presidency source said.

"We urge the authorities to release him immediately and to resist from all harassment of opposition members," added the source for the EU's rotating presidency, held by Slovenia till the end of the month.

EU foreign policy chief Javier Solana also condemned the arrest and called for Tsvangirai's immediate release.

"Less than a month before the second round of the presidential election and with the violence continuing, Mr Tsvangirai's arrest will heighten further the fears of the Zimbabwean people and the international community about the conditions under which this poll, crucial for the future of Zimbabwe, will be held," said a communique issued by Solana's office.

EU Development Commissioner Louis Michel said he was "deeply concerned by the reports of the detention of some leaders of the Zimbabwe opposition...

"We are endeavouring to clarify the situation and obtain further information on the ground," he said.

"The situation raises the wider concern that appropriate conditions be in place ahead of and during the second round of Presidential elections on 27th June."

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