By James Copnall
BBC News, Nouakchott
Mr El-Wakef was reportedly suspected of trying to flee the country
Mauritania's former prime minister has been re-arrested, a few days after being freed by the new military rulers.
Yahia Ould Ahmed El-Waqef was heading to the city of Nouadhibou for a rally against the 6 August coup when he was stopped, his supporters say.
Mauritania is now governed by a military council after the fifth coup in the country's short history.
The ousted President Sidi Ould Cheikh Abdallahi is still in the hands of the military.
Mr El-Waqef was taken away by a police inspector in a car which headed towards the capital Nouakchott, according to his supporters.
Since then the political parties which support the ousted government have had no news of Mr El-Waqef.
He was only freed 10 days ago, after being held by the military, which took over the country earlier this month.
A source close to the military told the BBC the former prime minister was suspected of trying to flee the country.
Nouadhibou is near the border, and the source said the authorities believed Mr El-Waqef was trying to escape to Morocco.
The majority of deputies who support the coup have opened an extraordinary parliamentary session, and they intend to investigate corruption in the former government.
In particular, an emergency plan to deal with the crisis in rising food prices - to which Mr El-Waqef was closely linked - is being examined.
The source said he believed Mr El-Waqef was attempting to flee before the investigation began.
However, the former prime minister's supporters accuse the coup leaders of cracking down on liberties, and see his arrest as simply further proof of this.