Harare - Banks, factories and stores were forced to close across Zimbabwe on Wednesday as workers heeded the call of the country's largest labour federation to strike in protest of the government's decision to dramatically increase petrol prices.

Labour union officials say that with last week's tripling of the petrol price workers struggle to afford the commute to work.

Tensions ran high in the troubled southern African country as the first day of the planned three-day strike took hold.

Police manned road blocks on main highways and troops patrolled impoverished townships in eastern Harare. A military helicopter flew over the capital.

The Zimbabwe Congress of Trade Unions, the umbrella grouping of trade organisations which called the strike, is closely affiliated with the main opposition party.

Four union officials have already been arrested for helping plan the strike and there were fears more retribution could follow.

The increasingly authoritarian government has declared the strike illegal under stringent security laws that have outlawed any anti-government demonstrations.

An anti-government strike last month succeeded in shutting down businesses across the country for two days and was followed by a massive crackdown by the authorities. Hundreds were arrested and many were beaten.

Zimbabwe is suffering its worst political and economic crisis in decades, with massive shortages of food, gasoline, medicine and essential imports.

On Wednesday, bus stops and parking lots were virtually deserted and traffic was light downtown.

Factory owners in Bulawayo, the country's second largest city, said about three-quarters of the city's factories had closed.

State radio reported that most businesses were also closed in the eastern cities of Marondera and Mutare.

The state radio also said that thousands of people eager to go to work in Harare were hampered by shortages of transportation and shops that shops in the capital locked out workers who managed to arrive at their jobs.

In the eastern Harare township of Mabvuku, lines of commuter waiting for regular transportation dispersed after only a few buses were operating, residents said.

Main supermarkets were closed. Some shops opened one of their entrance doors but left iron barriers and shutters closed.

There were no immediate reports of violence or arrests.

labour union officials said they were determined to follow through on the strike, despite the dangers.

"We know they will arrest us but we are not worried since we are fighting a genuine cause," said Wellington Chibebe, secretary general of the Zimbabwean Congress of Trade Unions.

The hike in petrol prices has more than doubled commuter and bus fares.

Workers now spend up to three quarters of their earnings on transportation.

Chibebe said strikers were demanding an immediate reversal of the new petrol prices.

The opposition has promised to step up a campaign of strikes and demonstrations in the name of bringing democratic reform to Zimbabwe. - Sapa-AP