DHAKA, Bangladesh—Opposition leaders in Bangladesh on Monday said they would continue with antigovernment protests that since their start earlier this month have at times turned deadly, with firebombs thrown at buses and clashes between armed partisans killing at least 25 people.

The defiant call by the Bangladesh Nationalist Party and its allies followed a move by the police to release opposition leader Khaleda Zia, who had been kept confined to her office in central Dhaka by security forces for 15 days.

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