President Mugabe has vowed to crush the MDC

Mr Tsvangirai, who heads the Movement for Democratic Change (MDC), was taken from his house at about 0530 local time (0330GMT) on Friday, and released several hours later.

The MDC leader said he had been accused of failing to hold a licence for a walkie-talkie that he used to communicate with his personal security guards.

"I don't think it has anything to do with a licence, it's just harassment," Mr Tsvangirai told the French news agency AFP.

Threat to Mugabe

Mr Tsvangirai's arrest came as President Robert Mugabe launches his campaign to be re-elected next year. Mr Tsvangirai is expected to be his main opponent in the poll.

He was held at Harare's central police station in connection with a walkie-talkie found at his home, said MDC spokesman Learnmore Jongwe, who added that a licence was not necessary.

The MDC said on Thursday that Mr Tsvangirai's house had earlier been searched by armed police and that three of his guards had been arrested.

The European Parliament has called for sanctions to be imposed on Zimbabwe because of what it called the deterioration of the rule of law and increasing human rights abuses.

Elections

The BBC's Southern Africa correspondent, Barnaby Phillips, says the MDC will see the incident as part of a pattern of harassment by the government as Zimbabwe enters elections - due in March.

Mr Mugabe has vowed to crush the MDC and has repeatedly accused the party of terrorist activity.

"Violence is not just happening, it in fact has been deliberately hatched at the centre of the MDC and by its patrons and principals overseas... This is a real physical fight and we have to prepare for it," he said in a nationally televised speech on Thursday.

But human rights groups in Zimbabwe say it is the government that is responsible for the rising tide of violence in the country.