To prevent people from gather on a place and make plans to fight against britishers, they imposed martial law.
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In 1919 gandhiji decided to launch satyagraha against the rowlatt act passed bu the Imperial legislative council. So rallies were organised, workers went on strike and shops close down. Police in amritsar fired upon a peaceful procession, provoking widespread attacks on banks, post office etc. Therefore martial law was imposed and general dyer took command
to have control on the activities of the leaders