NEW DELHI — The public suicide of a farmer at a political rally on Wednesday appeared to add momentum to the opposition to Prime Minister Narendra Modi’s land policies.

The farmer, apparently from arid Rajasthan State in western India, climbed a towering neem tree near the stage at the rally in Delhi, where politicians were speaking out against a new land bill. The farmer tied a noose out of a white scarf, placed it around his neck and stepped off a branch to his death. His body hanged in view of scores of shocked rally participants until attendants managed to free it from the tree, witnesses said.

“I saw him alive, and a few minutes later, I saw him dead,” said Jogender Deshbhakt, 30, an activist with the Aam Aadmi Party, which organized the rally and which governs in Delhi State. Mr. Deshbhakt said that when he saw the farmer tightening the noose, he climbed up the tree to try to stop him, but did not reach him in time.

He criticized the police officers on the scene for failing to act, calling them “silent spectators.”

The Aam Aadmi Party organized the rally for farmers to oppose the central government’s proposed legislation that would make it easier for the government and private companies to acquire land for certain industrial projects.

Opposition parties have marshaled the suffering of farmers affected by crop losses and unseasonal rains to considerable effect in fighting the bill. The public suicide seemed likely to add fuel to the opposition.

Television stations showed video clips of the suicide, and a note that witnesses said was found in the farmer’s pocket circulated swiftly on social media.

“Friends, I am a farmer’s son,” the note said. “I’m thrown out by my father because my crop is destroyed. I have three children. Please tell me, how do I go back to my home?”

It concluded with a nationalistic slogan popularized during the 1965 war between India and Pakistan: “Hail the soldier, hail the farmer.” It was signed Gajender Singh.

The authenticity of the note could not be independently verified.

A police officer at the scene said that he was unable to comment on the man’s death, pending an investigation.

Mr. Modi expressed his condolences on Twitter on Wednesday, writing that the death “has saddened the nation” and that “at no point must the hardworking farmer think he is alone.”

Farmers who had gathered in Delhi for the rally voiced shock at the death and concern over their own economic problems.

Mahant Singh, 28, from Uttar Pradesh State, said he had lost most of his wheat crop this year because of unseasonable rains. “Now we are in real distress,” he said. “The government may give us some compensation of a few hundred rupees. How will my family eat for the rest of the year?”

India’s political parties reacted to the farmer’s suicide by pointing fingers. The chief minister of Delhi, Arvind Kejriwal, a vocal opponent of the land bill, referred to the farmer in a speech shortly afterward at the rally.

“He came here from Rajasthan, tried to end his life hanging from a tree to bring attention to himself and his condition, to bring it to the attention of the whole country — that’s why we are all gathered here today,” Mr. Kejriwal said.

He criticized the Delhi Police, who are controlled by the national government, not the state authorities. “We kept urging the police from the stage repeatedly, ‘Please save him, please save him!’ ” he said.

Rahul Gandhi, of the Indian National Congress party, rushed to the hospital where the farmer’s body was taken, telling reporters on the way, “The Modi government is a government that helps corporates,” while “the farmers and laborers of this country have built this country.”

He added, referring to Mr. Modi’s ruling Bharatiya Janata Party by its initials, “We’re not just going to let the B.J.P. government take the land of our poor farmers.”

Politically motivated suicide, if that is what the farmer’s death proves to be, is rare but not unheard-of in India. In the 1990s, scores of young people committed suicide to protest the increasing use of caste quotas to fill central government jobs. And in the agitation to create a new state, Telangana, out of part of Andhra Pradesh, a number of political suicides were reported.