Protesters in south Sudan responded to the arrests by torching the offices of the country's ruling National Congress Party (NCP), a southern government official said.
Salva Kiir, the president of South Sudan, condemned the arrests, saying they broke the Comprehensive Peace Agreement (CPA) of 2005 that ended a devastating 22-year civil war between north and south.
"These arrests are not only provocative but unjustified, because the interim national constitution of the Sudan and the CPA allow for peaceful and democratic procession," Kiir said, who is also first vice-president in a national unity government in Khartoum.
Police had announced that the electoral reform demonstrations would be considered illegal.
However, several hundred opposition protesters marched on the streets of Khartoum and Omdurman, waving placards and chanting: "We want our freedom."
Police, who initially trailed the demonstration, used tear-gas to disperse them and beat protesters with batons.
Also among those arrested were Barmina Awrial, Khartoum's state minister for health, and Siddig al-Turabi, son of Hassan al-Turabi, the veteran opposition National Islamic Front leader, along with more than 70 others.
Security forces blocked roads leading to parliament, with a heavy presence in key areas. They also closed the bridge to Omdurman, which lies on the west bank of the Nile river across from Khartoum.
"The security committee for Khartoum state has met and decided that the protest is illegal," Mohammed Babikir, a senior police officer said on state television late Sunday.
"Whoever takes part in this demonstration will be breaking the law," he said, shortly after Arman had told journalists he expected "thousands of our people" to take part.
|The vote will be Sudan's first since 1986 [AFP]
The SPLM and the ruling NCP of Omar al-Bashir, the president of Sudan, have failed to agree on democratic reforms ahead of elections next April.
They have also yet to agree on a law for the south's referendum scheduled for January 2011.
The national vote will be the first in Sudan since 1986, three years before al-Bashir toppled a democratically elected government in a bloodless military coup.
The SPLM and around 20 opposition groups called for a "peaceful protest" to exert pressure on the NCP.
Khartoum state announced the closure of schools on Monday and a day off for public employees to underline the government's "engagement ... towards democratic reform" and to aid voter registration.
Reform and changes to the election law were key aspects of the 2005 CPA.