When the Rowlatt Act came into law, Gandhi proposed that the entire country observe a hartal, a day of fasting, prayer, and abstention from physical labor, in protest against the injustice of the repressive new law. The response was overwhelming–on April 6, 1919, millions of Indians simply did not go to work, and for twenty-four hours (agonizing hours for the British) India simply ground to a halt. But Gandhi had pushed too far too fast, and turmoil engulfed the country. As he traveled around the country, the British arrested him, provoking angry mobs to fill the streets of India's cities. As violence swept through the country, Gandhi–to the amazement of many–ordered the mobs to return to their homes, and called off his campaign. If satyagraha could not be carried out without violence, he declared, it would not be carried out at all.

His appeal, however, came too late. In Amritsar, capital of the region known as the Punjab, the British authorities had panicked at the appearance of the huge crowds on April 6, and had deported the local Hindu and Muslim members of the Congress. The mobs turned violent, and Brigadier-General Reginald E.H. Dyer was summoned to restore order. Dyer proclaimed martial law, banning public meetings and instituting public whippings for Indians who approached British policemen. On April 13, perhaps unaware of the prohibition on public meetings or perhaps defying the British, a crowd of more than ten thousand gathered in the Jallianwalla Bagh, an open space, surrounded by buildings, in the center of Amritsar. Dyer arrived at the head of his troops, and, without warning, ordered his machine-gunners to open fire. As Winston Churchill later put it, the crowd "was packed together so that one bullet would drive through three or four bodies . . . (the people) ran madly this way and the other. When the fire was directed upon the centre, they ran to the sides. The fire was then directed upon the sides. Many threw themselves down on the ground, and the fire was then directed on the ground. This was continued for eight or ten minutes, and it stopped only when the ammunition had reached the point of exhaustion." The official British tally for the bloody afternoon was 379 killed and 1,137 wounded.

The British public were nearly as horrified as their Indian subjects. Dyer resigned in disgrace, and the London government repudiated his conduct vociferously. But the damage had been done. The Amritsar Massacre had the effect of pushing moderate Indian politicians, like Gandhi, toward outright rebellion, and it created a climate of hostility between British and Indians that would fester throughout the twenty-five-year march to independence. The old ideal of a benevolent, liberal British Empire lay shattered.

After many delays, the British finally allowed Gandhi to make his way to Amritsar. All along his journey, cheering crowds greeted his progress; once he arrived at the site of the massacre, he commenced his own investigation into the events of April 13. The report, when it was finally produced months later, differed little from the official account of the incident, but his work on it drew him into closer contact with a number of prestigious Indian politicians, including Motilal Nehru–the father of Jawaharlal Nehru, who would become Gandhi's great ally and the first Prime Minister of India.

After his work at Amritsar was done, Gandhi proceeded to the Muslim Conference being held in Delhi. The chief item for discussion was the feared suppression by the British of the Caliphs of Turkey, who Muslims considered to be the heirs of Mohammed and the spiritual heads of Islam, as part of the effort to restore order after World War I. Such a suppression would be a slap in the face to the Muslim population of India, and Gandhi urged them to respond. A simple boycott of British goods would not suffice, he asserted: if the British did insist upon the elimination of the Caliphate, Indian Muslims should refuse to cooperate with their rulers entirely, in matters ranging from government employment to taxes. But in the meantime, Gandhi urged caution, since the peace terms with Turkey had not yet been published.

Indeed, in the months that followed, Gandhi found himself supporting the British, who had recently passed the Montagu-Chelmsford Reforms, aimed at laying the foundation for constitutional self-government in India. For the increasingly nationalistic Congress leaders, these reforms did not go far enough, but when Gandhi threw his support behind the initiative, they had no choice but to go along–his fame was so great that the Congress could accomplish little without him, and he quickly became the body's guiding spirit. In any case, the period of rapprochement with the British did not last long. The British were clearly determined to abolish the Caliphate, and still enforced the odious Rowlatt Act with vigor. In April 1920, Gandhi accepted the (largely symbolic) presidency of the Home Rule League, and shortly thereafter, he asked all Indians, Hindu and Muslim, to join him in "non-cooperation" with their rulers beginning on August 1. He himself set the example; on August 1 he returned the medals that he had received for valorous service in the Boer War.

The period of "non-cooperation" that began in 1920 saw Gandhi moving all across India, encouraging people to give up their Western clothing and British jobs. In his wake came other volunteers, who worked to turn the Congress into something it had never been before–a grass-roots organization, with countless local units. This effort, despite the interference of the British authorities, was a great success, and in September of that year, Gandhi passed an official constitution for the Congress, with a system of local units working under two national committees. It was a structure that would endure for decades. Over the next year, Gandhi continued to crisscross the country, unmolested by the agents of the new Viceroy, Lord Reading, who was reluctant to arrest the popular figure. By 1922, Gandhi had deemed that the time was right for a move from non-cooperation into outright civil disobedience. However, a terrible atrocity intervened in his plans: a mob in the city of Chauri Chauri hacked the local constables to death. Gandhi, who had brought India to the brink of revolution, now was sufficiently horrified by the atrocity that he abandoned his plan for civil disobedience and retired into a period of fasting and prayer. It was during this period, in March of 1922, that Lord Reading finally gave the order for his arrest on a charge of sedition.