(Adds U.S. condemnation, paragraphs 4; 16-18)

By Nelson Banya

HARARE, Oct 16 (Reuters) - Zimbabwe's main opposition said on Tuesday President Robert Mugabe's government was escalating a violent crackdown against its members, but said it would not walk away from talks with the ruling party.

Nelson Chamisa, spokesman for the opposition Movement for Democratic Change (MDC), told journalists the opposition was worried by increased cases of violence against its supporters.

"We have witnessed an escalation in the number of assaults, violence and intimidation against our members, particularly in the rural areas but also in the urban areas," Chamisa said.

The United States -- which along other Western powers is accused by Mugabe of trying to oust him from power in favour of the MDC -- condemned the latest crackdown as "more of the same".

Mugabe's ruling ZANU-PF and the MDC are holding talks mediated by South African President Thabo Mbeki, as part of a regional drive to end Zimbabwe's political and economic crisis.

The negotiations yielded a compromise constitutional amendment last month, which would allow Mugabe to pick a successor if he retired mid-term but the president's powers to choose members of parliament have been curbed.

Chamisa said the fresh attacks were worrying.

"This is particularly worrying if you consider the process going on in South Africa. There's no use to be in talks in Pretoria and at war here at home."

He said the MDC would not pull out of the talks to protest against the government crackdown. Opposition officials have recorded over 4,122 political violence cases since January.

"We know ZANU-PF is trying to jeopardise the process, but we want the process to benefit the nation," said Chamisa, adding that several MDC meetings had been cancelled or interfered with by the police.

The government has also accused the opposition of carrying out and fomenting violence.

Zimbabwe's tough security laws require political parties to notify the police when holding political gatherings, but the opposition says police often ban the meetings without reason.

On Tuesday, anti-riot police broke up a march by the National Constitutional Assembly (NCA), a pressure group which is pushing for a new constitution.

A Reuters correspondent saw a small group of placard-waving protesters singing revolutionary songs and denouncing the recent constitutional changes, before they broke up when police arrived on the scene, warning them to disperse.

No arrests were reported and NCA officials were not immediately available for comment.

U.S. State Department spokesman Tom Casey strongly condemned the latest crackdown on the opposition in Zimbabwe.

"It is unfortunate that the government of President Mugabe has continued to place obstacles in the way of the opposition and just average citizens being able to express their views," Casey told reporters in Washington.

"Unfortunately it appears to be more of the same with regards to President Mugabe and his refusal to allow people to exercise some of their basic human rights," he added.

Mugabe, 83 and in power since independence from Britain in 1980, denies charges of human rights abuses and mismanaging a once vibrant economy, now with world's highest inflation rate. (additional reporting by Sue Pleming)

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