Police stormed the home of Ousainu Darboe, leader of the main opposition United Democratic Party (UDP), rounding up supporters and party officials who had gathered there, witnesses told Reuters by telephone from the capital Banjul.
Darboe was among those arrested, family members said.
He had earlier held a news conference where he demanded answers from the authorities amid reports that Solo Sandeng, the party's National Organising Secretary, had been tortured to death while in detention.
"I'm ready to die. I'm not going to ask for police permission (to demonstrate). I want to see the body of Solo, dead or alive," Darboe said, according to a witness who was present.
The government of the tiny West African nation had acknowledged making arrests following Thurday's demonstration. Police sources confirmed Sandeng had been among those detained.
The small protest, which called for election reforms and free speech protection, was a rare act of defiance and occurred while President Yahya Jammeh was in Turkey attending a summit of Islamic countries.
Government and security officials were not available to comment on Saturday, but Amnesty International said that, according to information it had received, Sandeng had died.
"The tragic death in detention of Solo Sandeng must leave no space for impunity. The authorities must conduct an immediate, thorough and independent investigation," said Sabrina Mahtani, Amnesty International West Africa researcher.
Amnesty said that another detained UDP member, Fatoumata Jawara, was also believed to be suffering from serious injuries.
Jammeh, who seized power in a bloodless coup in 1994, has made headlines for eccentric proclamations, including a claim to have invented a cure for HIV/AIDS and his recent surprise decision to make Gambia an Islamic republic.
But he is also regularly denounced by rights groups and foreign governments for ruthlessly stamping out political dissent in the nation of 2 million people, which is a popular beach destination for budget-minded European tourists.
The former military man, who once told a reporter he could lead Gambia for "a billion years", is expected to extend his rule in elections in December.
He has scrapped term limits from the constitution, and the regional bloc ECOWAS refused to send observers to the last elections in 2011, citing intimidation of the opposition and the electorate.