Beijing (CNN) Six Marxist university students who wanted to spend International Labor Day working alongside regular laborers have vanished in China, the latest in a series of disappearances and detentions of student activists.

An activist group linked to the six missing students announced their disappearance in a statement released Tuesday. CNN tried to reach out to the students but found their phones were switched off.

Since August 2018, left-wing students, including those from Beijing's prestigious Peking University, have been detained across China apparently over their involvement in worker protests in different parts of the country.

The Chinese government is highly sensitive to attempts to organize protests or support what it considers subversive messages. In most countries, International Labor Day is often marked by parades and demonstrations held by trade unions, but in China is generally regarded as a holiday, despite the country's Marxist roots.

The latest round of disappearances appear to have taken place in recent days, ahead of a speech Tuesday by President Xi Jinping in honor of the 100th anniversary of the May 4 student protests.

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