Mr Obasanjo's second term in office ends next year

The Economic and Financial Crimes Commission (EFCC) said it was following up claims that MPs had been offered up to 50m naira ($390,000; �206,000).

The National Assembly is discussing a bill to change the constitution to let presidents serve third terms in office.

Many MPs oppose the bill and it may not get the necessary two-thirds support.

President Olusegun Obasanjo has not said whether he would like to remain in office after his current term expires next year.

Selective

Last week, MP Uche Onyeagucha told the BBC that he had been offered a plot of land in the capital, Abuja, if he would support the bill.

The EFCC has urged those with evidence of wrong-doing to produce it.

Vice-President Atiku Abubakar would like to stand for president

The BBC's Alex Last in Lagos says allegations of bribery are not new in Nigeria - it is ranked as one of the most corrupt countries in the world.

The EFCC was set up by the government as part of its much-vaunted anti-corruption drive, but critics say it has been too selective in its targets, particularly going after known opponents of President Obasanjo.

Given the immense importance of the bill to change the constitution, failure to launch an investigation would further damage the organisation's credibility, our correspondent says.

Vice-President Atiku Abubakar, who also wants the top job, has declared himself against the amendment.

He has helped Mr Obasanjo win two elections and has held the position of vice-president since 1999.

Some opponents of the constitutional change argue that the presidency needs to rotate among people from different regions and ethnic groups.