The leaders of the Brics emerging economies have signed a declaration stressing the importance of an "open world economy", in which all countries benefit from globalisation.
Brazil, Russia, India, China and South Africa also backed an "open and inclusive" multilateral trading system under World Trade Organization rules.
But they said the multilateral trading system faced unprecedented challenges.
Their comments come amid mounting trade tensions sparked by US tariffs.
The leaders have been meeting at the 10th Brics summit in Johannesburg.
"We reaffirm the centrality of the rules-based, transparent, non-discriminatory, open and inclusive multilateral trading system, as embodied in the World Trade Organization, that promotes a predictable trade environment and the centrality of the WTO," the declaration signed by the five leaders said.
However, they added: "We recognise that the multilateral trading system is facing unprecedented challenges.
"We underscore the importance of an open world economy, enabling all countries and peoples to share the benefits of globalisation, which should be inclusive and support sustainable development and prosperity of all countries.
"We call on all WTO members to abide by WTO rules and honour their commitments in the multilateral trading system."
The Brics summit is the first since US President Donald Trump placed tariffs on billions of dollars' worth of goods from around the world, in particular China.
He has promised further levies on $200bn (£150bn) worth of Chinese products in September.
Mr Trump also wants to cut the trade deficit with China – a country he has accused of unfair trade practices since before he became president.
Mr Trump made a big point on the campaign trail about cutting the country's trade deficits.
He is convinced that they hurt US manufacturing, and has said repeatedly while campaigning and on Twitter that the US must do more to tackle them.
Speaking in Johannesburg on Thursday, Chinese President Xi Jinping said: "We must work together… to safeguard the rule-based multilateral trading regime, promote trade and investment, globalisation and facilitation, and reject protectionism outright."
On Wednesday, he said there would be no winner in a global trade war.