Mahatma Gandhi’s letter was, in a way, an ultimatum. If the demands were not fulfilled by 11 March, the latter stated, the Congress would launch a civil disobedience campaign. Irwin was unwilling to negotiate. So Mahatma Gandhi started his famous salt march accompanied by 78 of his trusted volunteers. The march was over 240 miles, from  Gandhiji ashram in Sabarmati to the Gujarati coastal town of Dandi. The volunteers walked for 24 days, about 10 miles a day. Thousands came to hear Mahatma Gandhi wherever he stopped, and he told them what he meant by swaraj and urged them to peacefully defy the British. On 6 April, he reached Dandi, and ceremonially violated the law, manufacturing salt by boiling sea water.

Which movement is described above?