Tusk directed remarks toward U.S. President Donald Trump and Russia's Vladimir Putin, noting that they would be meeting later in the day in Helsinki. U.S. exports to China were only about $130bn a year ago.
The United States launched five separate World Trade Organization dispute actions on Monday challenging retaliatory tariffs imposed by China, the European Union, Canada, Mexico and Turkey following USA duties on steel and aluminium.
China has sworn to retaliate at each step.
"Our summit is not directed at any third party, " said Li.
"It is the common duty of Europe and China, but also America and Russian Federation, not to destroy [the global trade order] but to improve it, not to start trade wars, which turned into hot conflicts so often in our history", Tusk said in Beijing.
Tusk said the world needs trade reform and not confrontation.
Tusk urged China, the United States and other countries to avoid trade wars and reform the World Trade Organization, equipping it to combat forced technology transfers and government subsidies, complaints underpinning Trump's tariffs. "The data shows that global trade headwinds have yet to grip China's economy", Tom Rafferty of the Economist Intelligence Unit said, as quoted by the Financial Times.
"Today we are facing a dilemma: whether to play a tough game, such as tariff wars and conflict in places like Ukraine and Syria, or to look for common solutions based on fair rules", Tusk said.
Xi said China stands ready to work with the EU on the basis of mutual respect, fairness, justice and win-win cooperation, to make still further progress in the China-EU comprehensive strategic partnership and promote bilateral economic and social development and people's welfare. French President Emmanuel Macron had called in late May for talks on overhauling the WTO.
On 6 July, the United States imposed 25% tariffs on $34bn in Chinese goods, prompting Beijing to hit back with levies on the same amount of U.S. exports to China.
Li said China wanted to improve the authority of the WTO, but did not elaborate.
Xi noted that the Chinese side would always regard the European Union as an important force for stability and development of the global structure.
Chinese Ambassador to the European Union wrote in a Chinese newspaper editorial on Sunday before the talks: "I hope during the summit China and the European Union will consolidate consensus and trust... and send a joint message defending multilateralism, free trade and investment facilitation". Leaders failed to find sufficient consensus for a joint statement after meetings in 2016 and 2017. "The first clear effect can only be that we need to align ourselves even more closely in Europe". They met amid mounting acrimony over Trump's tariff hikes on goods from China, Europe and other trading partners.
President Trump and the U.S. Trade Representative's Office have said the China tariffs are meant to force that burgeoning country to change its trade policies and do more to prevent the theft of U.S. technology and intellectual property.