And prominent rights campaigner Najati Tayara was picked up in the central industrial city of Homs, said the Syrian Centre for the Defence of Prisoners of Conscience.
The arrest came a day after Tayara reported shelling and gunfire had rocked Homs, Syria's third-largest city and the focus of a massive military operation since Monday.
The SOHR said security forces "have launched a sweeping arrest campaign across several cities."
Also detained was Adnan al-Shugri, chairman of the city council of Banias, it said.
Activists, journalists, and intellectuals were also detained in Damascus and the cities of Aleppo, Homs and Latakia, as well as smaller protest centres.
An activist in Banias said the city was like a ghost-town on Thursday afternoon, with people afraid to go out for fear of arrest and with army checkpoints on every corner.
Gunfire could be heard in Ein al-Nabe district, sealed off by the army while security forces went in to conduct arrests.
The Syrian Revolution 2011, a Facebook group organising protests against the government of President Bashar al-Assad, called for a "Free Women Friday" in support of arrested women demonstrators.
Several female protesters have been arrested, particularly in Damascus and Banias, where women marched calling for the release of their detained relatives and an end to the army's assault on protest hubs.
Four women were killed during May 7 protests, activists said.
US Secretary of State Hillary Clinton on Thursday warned Syria it will face more international pressure over its crackdown on popular protests, behaviour she called "a sign of remarkable weakness."
Syria has continued with "a brutal crackdown" against pro-democracy protesters despite what she called overwhelming international condemnation, she said on the sidelines of an Arctic Council meeting in Greenland.
"There may be some who think that this is a sign of strength but treating one's own people in this way is in fact a sign of remarkable weakness," she said.
Clinton warned Assad that he faced "increasing isolation" over his government's actions, but gave no sign Assad himself would be sanctioned, as members of his regime already have.
"I think it's fair to say that we are going to hold the Syrian government accountable. Now how that happens and what the timeline is (is) something we are working on as we speak," said the top US diplomat.
Late on Wednesday, thousands of students defied the crackdown to stage a protest in Syria's second-largest city, Aleppo, before being dispersed by baton-wielding loyalists and security force personnel, a rights activist said.
At least 19 civilians were killed across Syria on Wednesday as troops and unidentified gunmen assaulted protest hubs, firing on some and encircling others with tanks, according to accounts by human rights activists.
Among the dead was an eight-year-old boy, the head of the National Organisation for Human Rights in Syria, Ammar Qurabi, told AFP.
And two Syrian soldiers were killed and five others wounded in clashes with "armed terrorist gangs" in Homs and Daraa, according to the state news agency SANA.
Between 600 and 700 people have been killed and at least 8,000 arrested since the start of the protest movement in mid-March, according to human rights groups.
Syrian authorities insist they are pursuing "armed terrorist gangs."
Protesters are demanding free elections, the release of political prisoners, constitutional changes that would strip the ruling Baath party of its domination of Syrian political life and remove restrictions on other parties and the media.
Last month, under pressure from the international community, Assad lifted nearly five decades of emergency rule but the iron-fisted crackdown on protesters has continued unabated.
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