By MacDonald Dzirutwe

HARARE (Reuters) - Zimbabwe's High Court will on Friday decide whether President Robert Mugabe's opponents can proceed with a planned march calling for electoral reform after police chiefs suggested they present a petition instead, an opposition official said.

Leaders from 18 political parties, including Movement for Democratic Change (MDC) leader Morgan Tsvangirai and former vice president Joice Mujuru are due to lead Friday's demonstration, which they expect to draw thousands of supporters.

MDC secretary general Douglas Mwonzora said a High Court judge will hear the case at 8 a.m. on Friday, hours before the planned march.

Police used teargas and water cannon on Wednesday to break-up a march by MDC youth supporters, who were protesting against economic woes and what they say is brutality by security agents.

Police commander for the Harare Central District, Chief Superintendent Newbert Saunyama, told protest organizers in a letter on Thursday that they could present a petition at the Zimbabwe Electoral Commission without marching, adding that the expected number of participants, 150,000, was too big.

"The crowd cannot be accommodated in the central business district of Harare as it interrupts both human and vehicular traffic," Saunyama said.

Home Affairs Minister Ignatius Chombo labelled opposition leaders "foreign agents" using protests to cause chaos in order to justify international intervention in Zimbabwe's affairs.

"Those regime change agents bent on trying to remove a democratically elected government will face the full wrath of the law," Chombo told reporters.

In the first large scale demonstrations Zimbabwe has seen since 2007, protests inspired by social media movements such as #ThisFlag led by pastor Evan Mawarire have erupted in the past months.

Protesters want Mugabe to fire corrupt ministers, scrap plans to introduce local bank notes and end cash shortages that have caused queues at banks.

On Friday, opposition groups want the government to ensure the electoral field is fair ahead of presidential and parliamentary votes due in 2018 and does not favor the ruling ZANU-PF party, as well as setting out a roadmap for the ballot.

Mugabe, 92, who plans to contest the vote, has chided the opposition for seeking his downfall through protests, saying his opponents are afraid of defeat at the ballot box. He denies opposition and Western charges of rigging previous elections.

Under Zimbabwe's security laws, organizers are required to notify the police of any demonstration seven days before the event, but the police routinely ban protests by the opposition.

(Reporting by MacDonald Dzirutwe; Editing by James Macharia and Alison Williams)