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BEIJING — Nine Hong Kong activists were arrested Monday and accused of plotting to set off bombs ahead of a crucial debate on a controversial election-reform plan, local media reported.

Five men and four women between the ages of 21 and 34 were detained in a series of raids following the discovery of explosives at a vacant former television studio, the South China Morning Post newspaper reported, citing police sources. A police source told the newspaper that the suspects belonged to a radical group.

Hong Kong police were not immediately available for comment.

The apparent arrests came after thousands of pro-democracy protesters brandishing yellow umbrellas gathered in a massive march Sunday to show their unhappiness with the government’s election reform plan. The measures promoted by China would restrict the future candidates for Hong Kong’s leaders to those chosen by a nominating committee approved by the communist government in Beijing.

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The election reform controversy triggered the Occupy Central movement that convulsed the city for months in 2014.

The local legislative body will begin debating the election package Wednesday, but with 27 pro-democracy lawmakers already saying they will oppose the measure it is unlikely it will win the two-thirds majority needed to pass.