Mr Sadiq was released last year after being held since the 1989 military coup which overthrew him. He had been keeping a low profile until he attacked the government at a religious rally in Omdurman on the feast of Eid al-Adha, two weeks ago.
Many Sudanese predicted that his address would lead to open discontent among the poor of the capital, Khartoum, who have seen the Sudanese pound plummet and inflation rise steeply in the past few months. The arrest of the former president suggests the authorities thought the same.
A month ago Western diplomats were optimistic that the Islamic fundamentalist regime in Sudan, led by Omar Hassan el-Beshir, was bending to Western pressure on a number of issues including humanitarian access to southern Sudan and Sudan's links to 'terrorist' groups backed by Iran and Libya. Khartoum, they said, had interpreted US intervention in Somalia as a warning to Sudan and rapidly complied with requests for access to starvation areas in the south.
Recently, however, Radio Omdurman, the state-controlled radio station, has warned President Hosni Mubarak of Egypt that his days are numbered, denouncing his visit to the United States and Britain. Predicting the downfall of Mr Mubarak, General Beshir said that Sudan was not involved in terrorism, 'except against the enemies of God'.
Meanwhile the Sudan People's Liberation Army (SPLA), the southern rebel movement which split into three factions last year, appeared yesterday to have found a new unity at talks in Nairobi. Representatives of the three factions agreed a structure for a reunited SPLA. But a spokesman for John Garang, the original leader of the SPLA, who refused to attend, denounced the deal as a 'hoax'.