Sudanese security forces briefly held prominent opposition figure Mariam al-Mahdi and other women on Thursday, witnesses said, the latest detentions in a crackdown on anti-government protests.

Mahdi is the spokeswoman for the opposition Umma party and daughter of Sadeq al-Mahdi, Sudan's last democratically-elected prime minster who was overthrown by the current government in a 1989 bloodless coup.

Witnesses said security officers detained Mahdi as she was preparing to take a petition to government offices in Khartoum appealing for the release of young men arrested in recent protests. She was released soon after.

They just put her in their car and drove around Omdurman and Khartoum and at last they dropped her in the Umma Party and she came to us, said, Sumaya Habbani the wife of senior party official Mubarak al-Fadil whose two sons have been detained since January 30 protests demanding the government's resignation.

I think they (security) just wanted to separate the gathering ... We want them just to release the boys or to give them a court date. They have done nothing. They just went to protest peacefully on the street.

She said she had not spoken to her sons since their arrest. Other women picked up with Mahdi were also released in different parts of the city, she added.

Young people have staged a series of small demonstrations in north Sudan in recent weeks against rises in food prices and human rights abuses, partly inspired by uprisings in Tunisia and neighbouring Egypt.

Internet sites and some protesters have also called for regime change. Police, armed with tear gas and batons, have moved in quickly to end protests, say witnesses. Dozens of activists, journalists and opposition remain detained.

Last year Mariam al-Mahdi was attacked by a police officer who beat her on the head and hands, and also broke her arm. No reason was given for the assault.

Vans full of police officers surrounded Mubarak al-Fadil's house on Thursday as the women gathered inside. Four women were later allowed to deliver the letter, members of the women's group said.