Brasilia, Brazil - Brazilian President Luiz Inacio Lula da Silva on Thursday slammed Donald Trump's threat to impose 50% tariffs on Latin America's largest economy as "unacceptable blackmail."

Lula's comments during a nationally televised speech were the latest in series of tense exchanges between the leaders, with the US president launching especially blistering attacks on the government in Brasilia.

Trump announced on July 9 his intention to slap steep tariffs on Brazil as punishment for what he termed a "witch hunt" against his far-right ally, former president Jair Bolsonaro.

The US has also said it is investigating Brazil's "unfair trading practices," and Brasilia said it was committed to negotiations.

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The tariffs on all products from Brazil would kick in on August 1 if Brasilia and Washington do not reach an agreement.

In his speech, leftist leader Lula slammed Brazilian politicians who back Trump's policies as "traitors to the homeland."

He said he would continue "betting on good commercial and diplomatic relations" but warned: "Brazil has only one owner: the Brazilian people."

Bolsonaro is facing trial over accusations he plotted a coup after his narrow 2022 election loss to Lula. If found guilty, he could face up to 40 years in prison.

Earlier on Thursday, Trump posted a letter addressed to Bolsonaro on his Truth Social platform in which he insisted Lula's government "changes course" and "stop attacking" his political ally.

"I have seen the terrible treatment you are receiving at the hands of an unjust system turned against you," Trump wrote to Bolsonaro.

"I have strongly voiced my disapproval both publicly and through our Tariff policy," he added.

Trump also said he was "concerned about the attacks on free speech" in Brazil and in the US.

He appeared to be alluding to the suspension in Brazil of Rumble, a video-sharing platform popular among conservative groups, over its refusal to block a user accused of spreading disinformation.

Trump's intervention in the Bolsonaro case has improved Lula's popularity, who has appealed for national unity in the face of US "interference."

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Unlike the tariffs Trump is slapping on economies around the world, the measures against Brazil were announced in openly political terms.

Brazil had not been among dozens of trade partners previously threatened with duties above a 10% baseline.

The US runs a goods trade surplus with Brazil, which said it had repeatedly requested that Washington point out areas of particular concern.

Brazil expressed "indignation" at the stiff proposed tariff in a letter addressed to US Commerce Secretary Howard Lutnick and Trade Representative Jamieson Greer.