Nigeria says it is investigating what it calls "serious breaches of security," in the wake of media reports that disgruntled military officers may have been plotting a coup.
A presidential spokeswoman would not say how many people are being investigated, only that the number is "considerable." Among the suspects are military officers and civilians.
Military sources have told news organizations that 28 people have been interrogated over the last several days.
The spokeswoman declined to say whether the alleged offense constituted a coup threat. But she stressed that there is no danger to President Olusegun Obasanjo.
Local media reports in Nigeria say the suspected coup plotters have been seeking the support of soldiers who are angry over late or unpaid salaries.
Soldiers in Nigeria have staged several military coups and numerous failed attempts to overthrow the government since the country gained independence from Britain in 1960.
Nigeria returned to civilian rule in 1999 with the election of President Obasanjo. But the president's popularity has been hurt by allegations of fraud over last year's presidential and parliamentary elections.
And ethnic and religious divisions remain high in certain parts of country. More than 10,000 people have been killed during civil unrest since Mr. Obasanjo came to power.