Suleiman, head of the Democratic Forces Front and the Sudanese Human Rights Group, was arrested on 2 July, shortly before he was to hold a press conference.
"This time the treatment switched 180 degrees ... with the room air-conditioned, television, good food and reading facilities," he said, comparing his arrest to previous detentions.
The press conference was called to unveil the "Khartoum Declaration", a document supporting the peace process between Khartoum and southern rebels. It also supported an end to one-party rule in Sudan.
Suleiman has said the text "supports and explains" a statement known as the "Cairo Declaration" issued after talks held in Egypt's capital in May by leaders of the northern and southern opposition.
Since President Omar al-Bashir came to power in 1989, political opposition, freedom of expression, assembly and association in Sudan have been severely curtailed.
The Political Act, which came into force in January 1999, supposedly relaxed the ban on opposition parties.
However, opposition parties not supporting the government are either banned or denied genuine political freedom.
The Democratic Forces Front is a non-registered political party advocating democracy in Sudan.