Reporters Without Borders criticised the government in Khartoum after the internal security agency closed Aljazeera's office in the Sudanese capital on Thursday, accusing the satellite television channel of biased reporting.
Police also detained the station's bureau chief, Islam Salih. Aljazeera has denied bias and defended its arrested employee.
"We call on the security services to authorise the immediate reopening of Aljazeera's bureau in Khartoum and to stop censoring this TV network," said the secretary general of Reporters Without Borders, Robert Menard, on Friday.
Police had raided Aljazeera's office on Wednesday and seized broadcasting equipment, although staff at the office produced legal documents and permits. The bureau chief was taken in for questioning.
Minutes before his arrest on Thursday, Salih informed his head office in Qatar the authorities had decided to close the Khartoum office, quoting Sudanese security forces.
Sudanese officials denied police had raided the offices and confiscated equipment on Wednesday, saying customs officials had merely taken a device as a routine registration check. But Reporters Without Borders rejected this explanation.
Sudan's leadership has been irked
by the station's critical coverage
"The confiscation was clearly motivated by displeasure with Aljazeera's coverage of Sudan," the media rights group said.
Widely watched in the Arab world, Aljazeera has in the past upset many conservative Arab governments with its reporting.
Human rights organisations have often condemned Sudan for cracking down on freedom of expression.
In its 2003 report on Sudan, Amnesty International said security forces continued to limit media freedoms. Amnesty added authorities unlawfully arrested journalists and editors and fined or suspended newspapers.
Last month, al-Ayam daily newspaper was forced to suspend publication after it was accused of harming Sudanese national interests and helping its enemies. The paper denied the allegations.