Which 1919 law did Mahatma Gandhi oppose with a nationwide satyagraha?

Mahatma Gandhi opposed the Rowlatt Act with a nationwide satyagraha in 1919.

The Rowlatt Act, formally the Anarchical and Revolutionary Crimes Act of 1919, extended emergency wartime powers after the First World War. It allowed the colonial government to imprison people without trial in certain cases and restricted normal legal safeguards. Gandhi regarded the legislation as unjust and organized a countrywide hartal and protest campaign.

The agitation marked one of Gandhi’s first attempts to lead a movement on an all-India scale. The protests became entangled with severe violence in some places, and the Amritsar massacre on 13 April 1919 intensified public anger against British rule.

The act is sometimes confused with later constitutional laws, such as the Government of India Act of 1935. Those measures concerned the structure of colonial government, whereas the Rowlatt Act focused on emergency powers and political repression.