Human Rights Watch also decried the situation in Syria, saying government security and intelligence officials arbitrarily detained hundreds of protesters, subjecting them to torture.
The reports came as Syrian President Bashar Assad ordered detainees released following meetings with religious leaders, the official Syrian news agency SANA reported.
The U.N. experts called on Syrian authorities to end the repression and engage in meaningful dialogue on reforms, the United Nations said in a release.
"Firing on peaceful crowds attending protests or funerals is by no means justified," said Christof Heyns, special rapporteur on arbitrary executions, one of 10 independent experts urging the Syrian government to end the crackdown.
Since protests calling for democracy began in mid-March, the violence has escalated, resulting in at least 200 deaths so far, the experts said.
"Live ammunition is being used outside the prescribed limits and in clear violation of international law. Firearms may only be used in self-defense or in the defense of others," Heyns said.
Citing reports that people are arming themselves to retaliate against law enforcement officials, he warned the situation could "easily escalate into widespread violence."
The experts, who report to the U.N. Human Rights Council, said the crackdown is continuing despite the government's promises of reforms and consultations to end the 48-year emergency rule.
In its report, "Activists and Journalists also Arrested and Mistreated," HRW said lawyers and journalists who endorsed the anti-regime protests are also among those being detained.
"There can be no real reforms in Syria while security forces abuse people with impunity," said Joe Stork, deputy Middle East director for the New York organization. "President Assad needs to rein in his security services and hold them to account for arbitrary arrests and torture."
The organization said it had interviewed 19 people who had been detained and several families of detainees in Daraa, Douma, al-Tal, Homs and Banyas. Footage received from activists showed some of the detainees released by the authorities with torture marks on their bodies, the organization said.
The majority of former detainees told HRW they had witnessed dozens of others being beaten or heard screams. Three children said they had seen other children being arrested and beaten.
Those former detainees said they were subjected to torture, including electric shock, cables and whips, and were held in overcrowded cells, and deprived of food and sleep for several days. Some said they were blindfolded and cuffed throughout their detention.
The majority said they were forced to sign confessions without being permitted to read them and also had to sign a pledge promising to refrain from participating in protests in the future, the organization said.