The protesters assembled on Monday on the 41st anniversary of the ruling Socialist Arab Baath Party's accession to power. Emergency law - which activists assail as an infringement of human rights - was imposed days after the 1963 revolution.
Police ordered the protesters to leave because they lacked permission to gather and arrested about six who refused to go, witnesses said.
Police detained about five local and foreign journalists but released them shortly afterwards, journalists in Beirut said. The Human Rights Association of Syria (HRAS) later said about 30 people were arrested and condemned the detentions as a "violation of the right of peaceful gathering and expression".
HRAS said Aktham Naise, the spokesman of the Committees for the Defence of Democratic Liberties and Human Rights, was among those arrested.
Amnesty International said the state of emergency had led to thousands of suspected political opponents being detained,
tortured and held incommunicado without trial or charge,
sometimes for over two decades.
"Others have been convicted and sentenced to lengthy prison terms after grossly unfair trials before military or state security courts," the rights group said in a statement.
Al-Asad released political
prisoners after his succession
It called on Syria to drop charges against 14 human rights activists arrested in August at a lecture marking 40 years of the emergency law and now on trial before a military court.
Sources familiar with government thinking have said Damascus would only lift the emergency law if the state of war with Israel - which occupies Syria's Golan Heights - ended.
Damascus is facing mounting pressure from the United States to cooperate with US policies in the Middle East and implement democratic reforms.
"Others have been convicted and sentenced to lengthy prison terms after grossly unfair trials before military or state security courts."
After succeeding his late father in 2000, President Bashar al-Asad freed hundreds of political prisoners and tolerated political debates that openly criticised Syria's autocratic
However, activists said Syria later cracked down on critics, jailing several dissidents on charges such as trying to undermine the constitution and inciting insurrection.