Street vendor Antonio Zuniga was picked up by police in Mexico City in late 2005 without a warrant, jailed and sentenced to 20 years for the murder of a man he had never met.

Zuniga's story is the subject of a new Mexican documentary Presumed Guilty, that its makers hope will fuel a growing movement demanding public trials in Mexico and an overhaul of the country's broken justice system.

Mexico's courts are under scrutiny as the army has rounded up some 80,000 suspected criminals in a violent drug war that has killed more than 14,000 people since late 2006.

Our justice system is medieval, said lawyer Roberto Hernandez, one of the film's creators, at the Morelia film festival in central Mexico earlier this month. We arrest without warrants, we process without judges and we convict without proof.

Presumed Guilty peels back the veil on this system and recounts how with the help of the lawyers who made the film he exposed the dishonest practices behind his conviction and won a retrial and exoneration.

Mexico, a major industrialized nation with modern financial, manufacturing, mining and telecoms industries, has long failed to reform its festering justice system.

Most suspects in Mexico are arrested without warrants and charged without any physical evidence. Trials are not public. Instead, judges issue rulings based on written transcripts, usually without ever seeing the people they convict.

The film received critical acclaim during its first showings in Morelia, as well as at the Toronto film festival last month. In Morelia, it won the best documentary prize. Its producers are in negotiations with major distributors.

Unlike the United States or Europe, there is a dearth of Mexican film or television that accurately portray court room dramas. Most Mexicans are unaware how their system works.

Mexico's Congress did approve reforms in 2008 to introduce fairer trials. But only two wealthier, northern states have begun to implement oral hearings, while the rest of the nation's courts remain shrouded in secrecy and riddled with corruption.

Such is Mexicans' frustration with the system that the filmmakers were cheered by hundreds of people who gathered at a free public screening in Morelia's central plaza last weekend. This captures what we live every day, that our laws are rotten, said Rosi Urtado, a 46-year-old teacher. Every trial should be filmed like this.

NO EVIDENCE, NO PROBLEM

Hernandez and another lawyer, Layda Negrete, spent years compiling some of the first independent statistics on Mexico's prison system earlier this decade.

After years of working toward reform, the pair married and were about to set off to begin graduate studies in California when they received a call from one of Zuniga's friends, pleading for help.

Zuniga, 26, sold video games in a poor, dangerous area of Mexico City. He was also a breakdancer who, coincidentally before his arrest, made a rap video about a youth framed by police.

Police picked him up the day after a gangland murder and he spent more than a year in prison, before getting in contact with the filmmakers who helped him win his release.

Hernandez and Negrete discovered Zuniga's first lawyer had a fraudulent license to practice law and managed to get a retrial, which they filmed and then edited with the aid of Australian filmmaker Geoffrey Smith.

His conviction had been based on the testimony of one witness, likely coached by police under the pressure of arrest quotas, and despite any connection to the murder victim or positive tests for gunshot residue. We don't have a culture that tells you what your rights are. So you end up in prison and you don't know what's going on, Zuniga said.

When Hernandez and Negrete were fighting for reform before they made their film, they were unable to convince high level officials that changes were needed in the courts.

Judges don't see the problem, said Negrete. That's why this film is so important. You can say 95 percent of people are put away without physical evidence, but seeing (Zuniga's) story is much more powerful.

Mexico Manufacturing