Puts profit above labour standards and the environment
Serves big business interests
Benefits richer countries, not developing nations
The protesters are threatening to paralyse formal proceedings, which begin on Tuesday.
Trade ministers, officials and bureaucrats from 130 countries intend to launch a new round of trade liberalisation negotiations.
Industrialised nations argue that free trade has led to greater prosperity around the world.
But a coalition of environmentalists and human rights activists, the Direct Action Network, said tens of thousands of people would swarm into Seattle and make it impossible for delegates to reach the conference centre.
Environmentalists dressed as turtles injected a note of humour
Nadine Bloch said her group was planning a non-violent stoppage, but added it had had training camps on civil disobedience over the last week.
She said she had been told to expect 25,000 to 30,000 US union members.
On Monday, a preliminary conference meeting got off to a rocky start as a security threat delayed a seminar between WTO officials and organisations representing labour, environmental and other interests.
Throughout Monday, central Seattle was awash with different groups representing a host of social issues - from religious organisations to those concerned by child labour practices.
In one of the most dramatic protests, two environmental activists dangled from a nine-storey-high crane by a busy highway and unfurled a banner accusing the WTO of being anti-democratic.
Some protesters want the WTO to be scrapped
In the evening, several thousand protesters attempted to form a human chain around the conference venue while WTO delegates attended a cocktail party inside.
The BBC's Andrew Walker in Seattle said the rallies were mostly good-humoured events, with music, an inflatable dolphin and people dressed as turtles parading through the streets.
Police in riot gear were deployed at key intersections but the crowd remained largely peaceful.
However one demonstration did turn violent when several anti-trade protesters attacked a McDonald's restaurant, smashing windows.
Protesters took over a disused building
Others, carrying a banner protesting against the genetic engineering of food, jumped onto the roof of a city bus.
The protests coincided with those led by Jose Bove, leader of the 40,000-member French Peasants Confederation, who handed out Roquefort cheese outside McDonald's in protest at the US imposition of a punitive tariff on French luxury goods.
Mr Bove says the US legislation is a tit-for-tat measure against France, which has refused to accept US beef that has been treated with hormones.
The BBC's Paul Reynolds says the WTO is having to take notice of what protesters have promised will be the "protest of the century" but the emphasis is still on free trade.
UN steps in
United Nations Secretary-General Kofi Annan, who will address the conference, is to tell activists that the WTO is not the right forum to debate social and environmental matters, according to a senior UN official.
The UN chief is expected to argue that if issues such as labour standards, human rights and the environment are included in international trade negotiations, they "will become a pretext for raising further barriers against developing countries".
Mr Annan will say that the UN is a more appropriate forum to tackle the social and environmental dimension.
The demonstrations are expected to continue all week.