Lula was among 11 heads of state gathered in Honduras for a meeting of the 33-member Community of Latin American and Caribbean States (CELAC).
It was also attended by China, which is seeking to replace the United States as the main political and economic influence in the region.
"Arbitrary tariffs destabilize the international economy and raise prices," said Lula, the president of Latin America's biggest economy.
"History teaches us that trade wars have no winners," he added.
On April 5, US trading partners were slapped with a 10-percent "baseline" tariff which remains in effect despite Trump backing down Wednesday on a host of other, more onerous duties announced last week.
Trump's tariffs -- some of them suspended and altered numerous times -- threaten economic disruption for CELAC economies.
Mexico is the United States' biggest trading partner and source of much of its car imports, while Brazil is its second-biggest provider of steel.
Gathered in the Honduran capital Tegucigalpa, CELAC leaders said unity of purpose was needed in these economically uncertain times.
"We cannot continue walking separately when the world is reorganizing," host President Xiomara Castro said.
"The United States is redrawing its economic map without asking which people are left behind," she added.
Mexico's Claudia Sheinbaum said these were "times of profound changes in global trade."
"Today more than ever is a good time to recognize that Latin America and the Caribbean require unity and solidarity among their governments and peoples and to strengthen greater regional integration," she told the summit.
For his part, Colombian President Gustavo Petro said CELAC members "should help each other... let's not fall into the trap of solving problems alone."
- Beijing conference -
While Washington is increasingly seen as a volatile associate, China has been making inroads in the region.
Two-thirds of Latin American countries have joined President Xi Jinping's trillion-dollar Belt and Road infrastructure program, and China has surpassed the United States as the biggest trading partner of Brazil, Peru and Chile, among others.
In Honduras, a Chinese delegation led by Qu Yuhui, Beijing's number two for Latin American Affairs, has been holding bilateral meetings with CELAC delegates since Monday -- including envoys from Argentina, Brazil, Colombia, Chile, Mexico, Venezuela and Cuba.
Beijing also plans to host a China-CELAC ministerial conference on May 13, to be attended by Xi.
"China is set to increase its influence in Latin America; it is a gift from the United States," Peruvian international relations analyst Francisco Belaunde told AFP.
"China wants to appear now as a reliable partner that is in favor of free trade, it wants to take advantage of the mess generated by Trump and the annoyance of all countries over these tariffs," he added.
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