The first world fair since Shanghai in 2010 is to be inaugurated on Friday with organizers predicting 20 million visitors over the next six months and a €10 billion boost to the Italian economy.
It has also triggered a "No Expo" movement which is planning a major student rally on Thursday and a May Day march on Friday that is predicted to draw 30,000 people. Further events are scheduled over the weekend.
Organizers of the protests insist they intend them to be peaceful but the Italian media has been awash with reports of anti-capitalist collective Blockupy and other groups with a record of violent action planning to take part.
Milan police carried out raids on premises associated with several radical groups on Tuesday and Wednesday, seizing fireworks, baseball bats, gas masks and material which could be used to make Molotov cocktails.
A German national was arrested for possession of explosive material and 26 other people, including 16 French nationals, are being investigated, AGI news agency reported.
Opponents of the Expo say it is being staged on the backs of thousands of young people working for free or for minimal wages on a work experience basis.
They are angry that public money has been diverted to the project at a time of economic hardship, that millions have been syphoned off through corruption, and that corporations such as McDonalds and Nestle are involved in an event with the challenge of feeding the planet as its central theme.
Milan mayor Giuliano Pisapia has asked for security reinforcements to help police the demonstrations amid predictions that trouble is virtually inevitable.
"Milan is waking up in a climate of tension and bad omens it has not witnessed in 30 years," La Repubblica commented in a reference to the late 1970s and early 1980s when Italy was haunted by the spectre of homegrown terrorism.