MILAN (AFP) - The opening of Expo 2015 was marred Friday by violent clashes between police and dozens of militants opposed to Milan's hosting of the world fair.

Police fired tear gas at stone-throwing protestors wearing balaclavas or gas masks who had smashed windows and torched several cars and bins during a "No Expo" march which organisers said had brought 30,000 people onto the streets.

Water cannons were used to put out blazes, including one at a bank branch where the wall had been graffiti-ed with the words: "You've skinned us, today you pay."

The violent scenes left parts of Milan's elegant centre looking like a battle zone, giving a bitter-sweet start to a six-month food-themed event Italy hopes will bolster its international standing and boost its struggling economy.

"The authentic face of Milan is the positive, noble and handsome one displayed to the world and the future at Expo's opening, not that of these vandals who confront the security forces, to whom the government is very grateful," Prime Minister Matteo Renzi said in a statement.

Police made at least 10 arrests, and a similar number of police were were injured in clashes that had been anticipated for weeks.

MIXED START

Thousands of visitors poured into the specially created Expo site on the edge of Milan.

After months of concern about delays, partly linked to a corruption scandal that hit the organisation last year, it appeared most of the key construction work for the Expo had been finished in the nick of time.

But there was grumbling from participants over organisational problems which meant many of the pavilions and exhibits were not fully operational.

Declaring the fest open, Renzi bullishly insisted Milan and Italy had defied the critics.

"They said we would never do it but today the Expo is a reality," Renzi said.

However, many participants complained.

Belgium's pavilion opened with only a food-themed diamond exhibition on display and none of the country's celebrated beer and frites (fries) to serve up.

"The beer is in a warehouse 500m from here but we couldn't bring it in because the site was sealed off for a security sweep until 3am this morning," said Sebastien Steven, the pavilion's restaurant manager.

"We feel a bit let down. There has been a lot of disinformation."

Bangladesh reported similar problems.

"We can't open because we don't have any staff," said operations manager MD Abdul Matin.

"Our display was to be mainly cooking based but the chefs and other staff were all due to come from Bangladesh and the Italians have not granted their visas."

An official at Ireland's pavilion said: "Problems? Just a bit. Anything to do with accreditation here has been a complete nightmare."

Ticketless visitors arriving by underground train faced 90-minute waits in the rain to get into the custom-built, 110-hectare site.

That was due to a combination of ticketing software problems and long queues for the airport-style security scanners.

Among those whose patience was tested were a mother and son from New Delhi who were supposed to be inside opening a stall selling Indian crafts for their non-profit organisation.

They told AFP organisers had been unable to provide them with security passes in time due to "backlogging" in the processing of participants.

"I have been going to world exhibitions since Montreal in 1967 and I have never seen anything like this. We're quite angry," said the woman who did not want to be identified.

Unable to have any of their craft items delivered, her son was carrying as many as he could in two large sacks.

"We've paid 200,000 euros (S$300,000) to have the stall for the six months and we are having to buy daily tickets for 39 euros each just to get in. It's ridiculous," he said.

CORRUPTION

Organisers insisted the problems would be ironed out quickly.

But the "No Expo" movement does not look like it is going away.

"Our generation does not want this corporate bull****," one activist, who gave her name as Annalisa, told AFP.

The protesters have been angered by Expo's reliance on volunteer workers, the involvement of corporations like McDonalds and Coca-Cola and a perception that much of the public money ploughed into the project has been lost to corruption.

Italy secured the rights to host the Expo seven years ago, long before Renzi came to power last year.

But he embraced it as an opportunity for the country to show the world that it is regaining its confidence and reinventing itself under his leadership.

The event plays to food-loving Italy's strengths with its nutrition and gastronomy-based theme of "Feeding the Planet, Energy for Life".

Organisers are predicting 20 million visitors over six months and a 10-billion-euro boost to economic activity.