(Adds EU Presidency statement)

BRUSSELS, June 6 (Reuters) - The European Union demanded on Friday the immediate lifting of a ban on work by aid groups in Zimbabwe, saying hundreds of thousands of people in the country depended on such assistance for their survival.

The bloc also expressed serious concern about the detention of opposition leader Morgan Tsvangirai for a second time whilst campaigning for a June 27 presidential run-off.

President Robert Mugabe's government suspended all work by aid groups on Thursday nearly a week after banning some from distributing food, accusing them of campaigning for the opposition Movement for Democratic Change in March 29 elections.

"This ban must be lifted right away," EU Aid Commissioner Louis Michel said in a statement.

"I am deeply distressed to think that hundreds of thousands of people who depend on aid from the European Commission and others for their very survival now face an even more uncertain future.

Michel said all humanitarian relief efforts were based on the principles of independence, neutrality and impartiality.

"Further clarification from the Zimbabwean authorities is needed on the claims of inappropriate actions by certain relief organisations so that humanitarian operations can be restored in full without further delay," he said.

Michel said the ban would have serious consequences and meant NGOs and other international relief agencies, many of which operate with European Commission funding, were no longer allowed to provide basic humanitarian care to many of the poorest people.

A separate statement from the Slovenian EU Presidency expressed "serious concern" about the detention of opposition leader Morgan Tsvangirai whilst campaigning in Zimbabwe.

Zimbabwean police detained opposition Tsvangirai on Friday after blocking him from reaching a campaign rally for the June 27 presidential run-off vote. He was released a few hours after being stopped by armed police at a roadblock.

The Slovenian statement said the obstruction of NGO field operations and harassment of diplomats were heightening fears both in Zimbabwe and around the world over the conditions in which the run-off vote would be held.

On Thursday, police stopped and held five U.S. and two British diplomats for several hours after they visited victims of political violence.

The European Commission is the most important aid donor to Zimbabwe and last year provided 91 million euros ($142 million) in humanitarian aid and other assistance. (Reporting by David Brunnstrom; editing by Philippa Fletcher)

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