The head of the United Nations, Kofi Annan, has urged rich countries to open their markets to the poor as five days of world trade talks got underway in Mexico.
Demonstrators have clashed with police
In a speech read out on his behalf, Mr Annan said unfair trade policies caused damage to "billions" of people.
World Trade Organisation (WTO) director general Supachai Panitchpakdi said the "eyes of the world were on the conference" and WTO members had to "deliver".
Several thousand demonstrators clashed with police as they tried to break through a barricade around the site of the WTO meeting.
Protestors reportedly threw chunks of paving stones at police, who responded with tear gas.
A South Korean man - a farmers' leader and long-time critic of the WTO - stabbed himself and died during the protests.
One of his friends is reported to have said it was an "act of sacrifice" to show his disgust
at the WTO and its policies.
'Bold' steps needed
The 146 member governments of the WTO are trying to hammer out a new global agreement on trade - one that balances sharply contradictory demands from rich and poor nations.
Mr Annan was not actually in Cancun, having travelled to a UN Security Council meeting in Geneva instead.
But in a speech read out to the meeting, he called for progress in the trade talks.
"I urge you to say 'no' to trade policies that
aggravate poverty, and 'no' to practices that undermine aid," his speech said.
"And I urge you to say 'yes' to bold but sensible steps that will revive the global economy."
Mr Panitchpakdi said it was vital that the WTO succeeded in making progress.
"The outlook for the world economy remains uncertain and, despite some encouraging recent signs, we are not yet on the road to sustainable recovery," he said.
"We face a choice here in Cancun. Either we continue to strengthen the multilateral trading system and the world economy or we flounder and we add to the prevailing uncertainties.
"The eyes of the world are on this conference and people will judge us by the choice we make.
"There is only one possible answer. We have to deliver on the first choice," he said.
As he opened the conference, about 30 demonstrators inside the hall held up pieces of paper denouncing the WTO as "anti-development" and undemocratic.
Time running out
Cancun is supposed to be a crucial step towards the target of a new global trade deal by the end of 2004.
But the WTO has so far missed almost every deadline it has set since talks on the deal were launched in 2001.
Agriculture - primarily the favourable treatment rich countries afford their farmers - has been the main sticking point.
TRADE AND GLOBALISATION
Key issues at the trade talks
But governments have also taken sharply differing positions on issues such as pharmaceuticals prices, steel tariffs and intellectual property.
Despite much public optimism this year, few now believe that Cancun will result in a definitive deal, and there are reports that a so-far unscheduled extra ministerial meeting is being planned.
US President George W Bush has this week telephoned his counterparts in Brazil, Pakistan, South Africa and India, lobbying for a compromise when they meet.
Developing countries say the rich world needs to keep the promises it made two years ago to cut tariffs.
They also want to see an end to the hundreds of billions of dollars in farm subsidies in Europe, the US and Japan.
The big developing countries, including India, China and Brazil, have put aside differences, and are planning to use the WTO's rule of consensus to their own advantage.
But the rich world says poorer countries must agree to broader legal and commercial reforms in return for any concessions on farming.
Power to the people
There seems little appetite for compromise, however, either among developing-country governments or the activists who support them.
Poorer countries, many of which are major agricultural exporters, realise that they have the power to derail a deal at Cancun or later, and are insisting on the WTO's commitment to reduce farm subsidies sharply.
About 30 demonstrators protested during the opening ceremony
In 1999, trade talks in Seattle were derailed by mass protests.
Since 2001, when talks restarted, protests have been muted.
But that may be changing, with up to 150,000 people expected to arrive in Cancun over the next few days.