"I heard that they issued an international arrest warrant for me. The powers that be didn't have to go to that length. They could have simply called me to face a judge and I would have turned up."
Several thousand supporters of Issoufou's Nigerien Party for Democracy and Socialism (PNDS) turned up at Niamey airport to welcome him and accompanied him home.
The case against Issoufou is the latest chapter in a political crisis that has seen Tandja extend his time as president, increase his powers and remove presidential term limits, a move that has been criticised at home and internationally.
"You know, dictatorial regimes don't like the idea of an opposition existing. They don't like the idea of them so they criminalise them and accuse them of terrible things like money laundering"
PNDS party vice-president
The Reuters news agency quoted a senior legal source as saying that the money laundering investigation had found "numerous transactions" of large amounts of money "from dubious origins" through Issoufou's accounts, leading to the charges.
Niger authorities say the investigation is aimed at rooting out corruption, but Issoufou's deputy called the charges were politically motivated.
"You know, dictatorial regimes don't like the idea of an opposition existing. They don't like the idea of them so they criminalise them and accuse them of terrible things like money laundering," said Bazoum Mohamed, the PNDS vice- president.
"Issoufou would never take part in shameful practice."
The PNDS, which had previously allied with Tandja, earlier this month boycotted an election to replace parliament after Tandja dissolved it in the run-up to an August referendum boosting his powers.
ECOWAS subsequently suspended Niger's membership from the West African bloc and the country is also facing a possible freeze in European Union development aid.