(Updates after release)

CAIRO, March 3 (Reuters) - Egyptian security authorities on Tuesday released an Internet activist who belongs to an anti-government protest group, a day after he was detained at his home, a security official and a human rights group said.

Bloggers and human rights activists said security agents seized Rami al-Sweisy, a member of the Sixth of April Youth, along with his computer, mobile phone and wallet.

It was the third detention in less than a month of an Internet activist in Egypt and came as the movement called for a national day of protest on April 6, the anniversary of clashes between police and workers in a Nile Delta textile town in 2008.

"He was released today," said Gamal Eid, head of the Arabic Network for Human Rights Information. "He is a bit shaken".

Egyptian authorities have escalated tactics against bloggers and Web activists in recent weeks even as the government freed opposition politician Ayman Nour, one of its most prominent critics. Nour's release had long been demanded by Washington.

Last month, security men detained Egyptian-German activist Philip Rizk and blogger Diaa Eddin Gad, who campaign for Palestinians in the Gaza Strip. They held both incommunicado. Rizk was freed after several days after an international outcry.

Gad remains held in an unknown location. Rights group Amnesty International said he was at risk of torture.

Eid said interrogators tried to convince Sweisy to leave the left-leaning Sixth of April Youth, formed after April 6 clashes in the Nile Delta town of Mahalla el-Kubra between police and workers demanding more pay to compensate for soaring inflation.

Three people were killed and more than 150 injured over two days of unrest in Mahalla, the culmination of more than a year of strikes by workers at a giant state-run textile factory.

Sixth of April Youth has since transformed into a more general anti-government movement, collecting members through the social networking site Facebook, which along with blogs has emerged as a major forum for government critics in Egypt.

The government has faced rising public anger over its enforcement of a blockade on the Hamas-run Gaza Strip, especially since an Israeli offensive in Gaza in January.

Separately, the Egyptian government detained 15 members of the opposition Muslim Brotherhood on Tuesday over accusations of belonging to an illegal group and possessing anti-government literature, security sources said. (Writing by Cynthia Johnston; Editing by Janet Lawrence)