Congress in Paraguay is set to meet to decide whether to accept President Nicanor Duarte's resignation so he can take up a seat in the Senate.
Mr Duarte resigned on Monday, two months before president-elect Fernando Lugo is due to be sworn in.
Opposition leaders say Mr Duarte's move is an attempt to preserve his political immunity and so avoid corruption charges.
If Mr Duarte succeeds, vice-president Francisco Oviedo will take over.
Mr Duarte's five-year term in office was not scheduled to end until 15 August, when he was to be succeeded by Mr Lugo.
He heads the left-wing coalition that won the April election against Mr Duarte's Colorado Party, ending its 60-yaer hold on power.
It was during these elections that Mr Duarte won his seat in the Senate. He has said he must resign the presidency in order to be sworn in as a senator on 1 July.
Opposition leaders argue that the constitution bans presidents from exercising other functions, and so Mr Duarte should never have been allowed to stand.
Four opposition parties are now planning to boycott the resignation session and make it inquorate.
They say they believe Mr Duarte wants to launch a comeback from within the Senate, as well as enjoying immunity from any possible prosecution for corruption - allegations which haunted his three predecessors after they left office.
However Mr Duarte has said that he is "determined to contribute to democratic governability, to serve the interests of this country".
"If my resignation is not voted upon, Congress will be shirking its duties," he added.
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