The activists were released about three and a half hours later, with the head of security in Khartoum, Omar Abdul Motaled, saying that all confiscated material would be returned.
"These people are free to continue their meeting anytime, anywhere," he said.
Among those detained were five European diplomats, correspondents for Reuters and for Radio-France Internationale and two members of a Zimbabwean non-governmental organisation.
"It is a brutal attack on civil society. It is illegal and unconstitutional," said Faisal Baqar, a member of the Sudanese Association Against Torture who had slipped out of the meeting just before the police arrived.
"They did not identify themselves," said one of the EU diplomats after his release. "They searched everyone including women. They took pictures and filmed us."
The participants included representatives of Amnesty International, the Khartoum Center for Human Rights and Environment and of refugee rights group.
EU Ambassador Kent Degerfelt expressed surprise with Khartoum's heavy-handed response to the meeting.
"I am very surprised because things seemed to be going in the right direction," he said.
"It just goes to show why Sudan was not the right place to hold this summit," said Reed Brody from US-based Human Rights Watch.
Sudan is hosting the two-day summit opening Monday of the African Union that is to consider Khartoum's bid to chair the 53-nation body.