Tsvangirai's Movement for Democratic Change (MDC) planned the march to press veteran President Robert Mugabe for a new constitution to guarantee parliamentary and presidential elections due in March are free and fair.

"He has been released without charge. The police wanted to know what he is planning to do today," lawyer Alec Muchadehama told Reuters by telephone.

MDC secretary-general Tendai Biti told Reuters that Tsvangirai was picked up at 4 a.m. (0200 GMT) from his home in a suburb of the capital Harare by plain-clothes officers. He was released about four hours later.

Muchadehama said police also picked up another MDC leader, Dennis Murira, who was also released later.

Mugabe has been in power since independence from Britain in 1980 and critics say he has used tough security laws to keep his opponents in check. Mugabe denies the charge.

The MDC launched a legal challenge on Tuesday to a police ban on the march.

Tsvangirai was last arrested in March 2007 along with dozens of opposition officials ahead of another planned march. He says police beat him up in custody but they deny this.

RIOT POLICE

The police had initially granted permission for Wednesday's march, which the MDC also called to protest against a crumbling economy blamed on government mismanagement as well as to press for a new constitution.

A Harare magistrate's court was expected to hear an application at 0800 GMT on Wednesday on overturning the ban, which police say was prompted by fears the demonstration would degenerate into violence and looting.

Witnesses said armed riot police had been deployed on Tuesday night in the volatile townships of Highfield and Budiriro and set up road blocks on streets into the city centre.

A Reuters correspondent saw police combing a park where the MDC planned to assemble for the march in central Harare at 0915 GMT. The police were armed with shotguns, teargas canons and batons and conducted random searches on passers-by.

Police also deployed at various strategic venues in central Harare, searching cars for weapons.

Zimbabweans have tended to shy away from demonstrations in recent years, mainly from fear of a heavy-handed response by security forces and were on Wednesday busy queuing for cash at banks while many went about their normal business.