Brig. Gen. Jake Malajacan, an ally of several opposition figures linked to jailed former President Joseph Estrada, turned himself in to an Army intelligence unit at Fort Bonifacio in Makati City where he is being held, Reyes told reporters.
Reyes said Malajacan had denied involvement in the alleged plot to topple Mrs. Arroyo which included the violent storming of Malacañang on May 1 by thousands of Estrada supporters.
"Legal process will have to proceed" against Malajacan, Reyes said, adding that the military was studying punishing the general since he was still in active service.
The Philippine National Police and the Department of Justice were preparing their own case against Malajacan, the defense secretary said.
Malajacan was among 10 figures linked to Estrada whose arrests were ordered after the rioting near Malacañang in which at least four people were killed and more than 100 wounded.
Two of those earlier arrested, opposition Sen. Juan Ponce Enrile and former Ambassador to the US Ernesto Maceda, have been granted provisional liberty after posting bail.
Two other alleged leaders of the failed plot, Sen. Gregorio Honasan, a former Army colonel amnestied for past coup attempts in the 1980s, and Panfilo Lacson, the former police chief of Estrada, emerged from hiding yesterday in time to join a miting de avance of opposition senatorial candidates.
Mrs. Arroyo assured the opposition Thursday that Lacson and Honasan, who like Enrile are running for senator, would not be arrested as the campaign for the May 14 elections entered the homestretch.
Malajacan was a classmate of Lacson and Honasan in the Philippine Military Academy. They graduated in 1971.
He is a known demolition expert who also figured in botched coup attempts during the administration of former President Corazon Aquino.
Armed Forces chief Gen. Diomedio Villanueva is coordinating with PNP chief Director General Leandro Mendoza for the filing of charges.
Reyes told reporters yesterday that the military had placed Malajacan under tactical interrogation to determine if he is also liable for violation of the Articles of War.
Malajacan turned himself in to Col. Arsenio Tecson, vice commander of the Philippine Army, after a series of negotiations with the Intelligence Services of the AFP.
"I have directed Chief Superintendent Nestorio Gualberto, director of the Criminal Investigation and Detection Group, to process whatever documents they have and possibly inquest Gen. Malajacan without necessarily violating the lifting of the warrantless arrest," Mendoza said.
A state of rebellion was declared by Mrs. Arroyo on May 1 as pro-Estrada rallyists rioted in the streets around Malacañang, paving the way for the arrest without warrant of suspected power grabbers. The declaration was lifted last Sunday.
When asked about the role of Malajacan in the failed Labor Day attempt to seize the palace, Gualberto said the PNP received reports that Malajacan provided men with firearms during the rally at EDSA Shrine in Mandaluyong City as well as the loyalist mob that stormed Malacañang.
"We are collating evidence and we have two witnesses. We hope to file a case for preliminary investigation by next week," Gualberto said.
Malajacan was assistant division commander of the 8th infantry division of the Philippine Army stationed in Catbalogan, Samar. He was also a former leader of the Rebulusyonaryong Alyansang Makabansa.