Hundreds of Panamanians marched on Thursday to mark the anniversary of a deadly uprising against U.S. control of the Panama Canal in 1964, with some protesters burning an effigy of President-elect Donald Trump who has threatened to retake the vital global waterway.

More than 20 Panamanians, many of them students, died during violent clashes across the country in January 1964, which escalated after U.S. security forces opened fire in response to mass demonstrations against the U.S. presence in the country and control of the canal. At least three U.S. soldiers also died.

The incident, remembered every January 9 as “Martyrs’ Day,” is regarded as paving the way for the eventual transfer of the canal to Panama in 1999. It also serves as a reminder of a bloody past that still dominates national feeling over the canal in Panama, at a time of rising tension with Trump.

“Today is a day to remember the sacrifice of our martyrs, but also to say to the world that Panama is sovereign and the canal is ours,” said Sebastian Quiroz, an 84-year-old retired unionist who was a student during the uprising.

The marching crowd chanted “spilled blood will never be forgotten” and “hands off Panama” as they approached the monument of the eternal flame, built to remember those that died in 1964. Earlier in the day President Jose Raul Mulino laid a wreath at the site in a formal ceremony.

Trump on Tuesday refused to rule out using military or economic pressure to seize control of the canal, an 82-km (51-mile) artificial waterway connecting the Pacific and Atlantic Oceans that is a core international shipping route.

The president-elect has criticized the cost of moving goods through the canal and derided Chinese influence in the area. China does not control or administer the canal, but a subsidiary of Hong Kong-based CK Hutchison Holdings has long managed two ports located on the canal’s Caribbean and Pacific entrances.

Panama has strongly rebuked Trump’s threats.

“The only hands that control the canal are Panamanian and that’s how it will continue,” Foreign Minister Javier Martinez-Acha told reporters on Tuesday.

Ivan Quintero, a 59-year-old university worker attending the march, said no government could take away what Panamanians had fought so long for.

“Mr. Trump has been very disrespectful in threatening to take the canal away from us,” he said. “He has to learn to show respect.”

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