Echoing calls made by other international human rights groups recently, Human Rights Watch asked Bashar al-Assad to release scores of political opponents detained in the past few months as he tightened his grip on power.
In a letter sent to President Assad on Monday, the group said: "These arrests appear to be tied solely to the exercise by these activists of their guaranteed rights to freedom of expression and association.
"To punish activists in Syria for the peaceful expression of their views or for peacefully associating with others is a clear violation of Syria's own constitution and its obligations under international law."
Assad, under pressure from Western powers over Syria's role in Lebanon, has largely ignored calls to improve Syria's observance of human rights. Rights campaigners say that about 1,500 political prisoners are being held without trial.
The government has intensified its crackdown on dissent since January, arresting opposition figures, journalists and students, and imposing long jail sentences on several of them, campaigners said.
"To punish activists in Syria for the peaceful expression of their views or for peacefully associating with others is a clear violation of Syria's own constitution and its obligations under international law"
Human Rights Watch letter
A security court, the decisions of which are final, sentenced an Islamist activist to 12 years' jail earlier this month for belonging to the banned Muslim Brotherhood and a politician sympathetic to the Kurdish minority to five years' hard labour.
Amnesty International said in a statement last week that even when those arrested are put on trial, they face courts that "suffer a gross lack of independence and impartiality".
Activists who have been using the internet to publicise Syrian human rights violations have been also jailed, said Reporters Without Borders, which defends the rights of journalists.
Recent arrests have included those of Mohammad Ghanem, the editor of a website that calls for democracy in Syria, and journalist Ali Abdallah and his son.
Reporters Without Borders said Massoud Hamid, a journalism student, has been in prison since 2004 for publishing photos online of a pro-Kurdish demonstration in Damascus.