Robert Mugabe has been in power for 22 years
The law had been approved by parliament using invalid procedures after an earlier rejection by MPs, the judges decided.
The legal ruling came as the UK Prime Minister, Tony Blair, said Britain would call for action to be taken at the forthcoming Commonweath summit against President Robert Mugabe.
Earlier, opposition leader Morgan Tsvangirai was reportedly detained for holding an "illegal meeting" with his party officials.
The General Laws and Amendments Act introduced new electoral restrictions:
'Support'
President Robert Mugabe has long accused the former colonial power Britain of backing the opposition Movement for Democratic Change.
"The actions of Robert Mugabe are completely undemocratic and wrong and dictatorial," Prime Minister Tony Blair told parliament in London on Wednesday.
"If the result does go against Mr Mugabe... we must be in a position to make sure the proper democratically elected government of Zimbabwe is supported," he said.
Morgan Tsvangirai has had another brush with police after a meeting with party officials was declared illegal.
The incident comes two days after Mr Tsvangirai was charged with treason over an alleged plot to kill President Robert Mugabe.
Mr Tsvangirai was confronted by police when he emerged from a house in a Harare suburb, MDC officials said.
"They held them up for about 30 minutes before letting them go. There were no charges", said MDC spokesman Percy Makombe.
Cancelled
He said police declared the gathering illegal under a strict new security law passed last month.
"Anywhere we go, they will say it's illegal," Mr Tsvangirai was quoted as saying afterwards.
With the election less than two weeks away, the MDC is also complaining that key campaign rallies this weekend have been cancelled by police although they had previously been authorised.
The opposition party was reported to be trying to reschedule rallies planned in the second city Bulawayo on Saturday, and in Harare on Sunday.
Mr Tsvangirai said 79 rallies had been disrupted or cancelled by the police since the new security law took effect in January.
Education
Reports of violence in the run-up to the election have continued.
An opposition member of parliament, Tafadzwa Musekiwa, said his home in Chitungwiza south of Harare was attacked by a stone-throwing mob during the night. No one was injured but windows were smashed.
Mr Musekiwa called it "blatant state-sponsored violence and intimidation".
But Namibian election observer Kaire Mbuende played down the reports of violence, which is usually blamed on supporters of Mr Mugabe.
"There is violence associated with the electioneering process coming from both sides of the political divide," said Mr Mbuende, a close ally of Mr Mugabe.