The Anarchical and Revolutionary Crimes Act, 1919 popularly known as the Rowlatt Act was a legislative act passed by the Imperial Legislative Council in Delhi on March 18, 1919, indefinitely extending the emergency measures of preventive indefinite detention, incarceration without trial and judicial review enacted in the Defence of India Act 1915 during the First World War. It was enacted in light of a perceived threat from revolutionary nationalist organisations of re-engaging in similar conspiracies as during the war which the Government felt the lapse of the DIRA regulations would enable. Passed on the recommendations of the Rowlatt Committee and named after its president, British judge Sir Sidney Rowlatt, this act effectively authorized the government to imprison any person suspected of terrorism living in the Raj for up to two years without a trial, and gave the imperial authorities power to deal with all revolutionary activities. The unpopular legislation provided for stricter control of the press, arrests without warrant, indefinite detention without trial, and juryless in camera trials for proscribed political acts. The accused were denied the right to know the accusers and the evidence used in the trial. Those convicted were required to deposit securities upon release, and were prohibited from taking part in any political, educational, or religious activities.

Mohandas Karamchand Gandhi, among other Indian leaders, was extremely critical of the Act and argued that not everyone should get punishment in response to isolated political crimes. The Act annoyed many Indian leaders and the public, which caused the government to implement repressive measures. Gandhi and others found that constitutional opposition to the measure was fruitless, so on April 6, a “hartal” was organised where Indians would suspend all business and fast as a sign of their opposition and civil disobedience would be offered against specific law. This event is known as the Rowlatt Satyagraha. Gandhiji named the Rowlatt Act as “black act”.

Source: https://en.wikipedia.org/wiki/Rowlatt_Act