Although acquittals caused by witnesses withdrawing statements are common in India , where there is no police protection in criminal trials, the killing of a young, middle-class woman struck a chord with the public.

“The media has been reflecting middle-class outrage; not just against politicians and their sons using muscle power,” said Shammy Baweja, the investigations editor at news magazine Tehelka. “They are angry with the judicial system and the men in uniforms for failing to protect ordinary people. I think this cry for justice is not just loud, it's also passionate and it's probably going to last.”

Politicians have also come under pressure. Sonia Gandhi, leader of the ruling Congress party, has asked the home ministry to introduce new laws on witness protection and recording statements following crimes.

The Indian police force is governed by an 1861 colonial act which human rights groups claim is responsible for widespread abuse. The Chief Ministers of states appoint police commissioners and can remove them without cause.

Mandeep Tiwana, a criminal justice expert in Delhi , said: “If you have political connections here; if you're rich and powerful, you can tamper with the whole system.

“There have been debates since the 1980s about tackling the influence of political elites over the police,” he said, adding that often the first thing a newly-elected Chief Minister does is to appoint a police commissioner sympathetic to his party. “You can put pressure on the police chief and that percolates down to the rank and file. If he resists, he can be removed.”

This, said Tiwana, makes the police reluctant to pursue the wealthy and influential. When political influence does not work, threats and bribes are commonly used. “There was a very strong perception that those accused in the Jessica Lall case were criminals. Manu Sharma was pretty notorious,” he added.

Vikas Yadav, also a politician's son and acquitted along with Sharma, is the prime suspect in an ongoing trial for a murder that took place in 2002. Again, witnesses have refused to testify.

Sabrina Lall, Jessica's sister, said in a recent interview that one of the main witnesses had extorted money from both the prosecution and the defence. “Money and power held sway, with the entire system succumbing to the power play of the rich,” she said.

Tehelka's Baweja added: “ This case is a catalyst for change. The public wants to see the police protecting people.”

However, some legal experts warn that the outcry may become a lost opportunity if politicians clamour for new laws instead of police reform. Nitya Ramakrishnan, a Supreme Court lawyer, said: “There is no point in talking about witness protection and new laws unless you tackle police corruption.”