They summoned her for talks, yesterday, a day after the army overthrew her caretaker government in a coup.
As the army moved to consolidate its grip on the country, its chief, General Prayuth Chan-ocha, 60, and due to retire later this year, has taken over the powers of prime minister, but it was not clear if he intended to hold on to the position.
He set out his plans for the country, saying reforms were needed before an election. But some Thais defied martial law to protest against the takeover.
Gen Prayuth launched his coup after rival factions refused to give ground in a struggle for power between the royalist establishment and Ms Yingluck's populist government that had raised fears of serious violence and damaged the economy.
"We have detained Yingluck, her sister and brother-in-law," a senior military officer said. The two relatives have held top political posts.
"We will do so for not more than week, that would be too long. We just need to organise matters in the country first," said the officer who declined to be identified.
He declined to say where Ms Yingluck was being held, but media said she was at an army base in Saraburi province, north of Bangkok. Soldiers detained politicians from both sides on Thursday after Gen Prayuth announced the military takeover, which drew swift international condemnation.
In what appeared to be a co-ordinated operation to neutralise possible opposition to the coup, the military summoned the ousted prime minister to a meeting and then banned her and 154 others, including politicians and activists, from leaving Thailand.
Ms Yingluck is the sister of Thaksin Shinawatra, a billionaire telecommunications tycoon turned politician who won huge support among the poor but not the royalist establishment, largely over accusations of corruption and nepotism. He was ousted as premier in a 2006 military coup.
Responding to the summons, Ms Yingluck arrived at an army facility at noon along with other politicians. Gen Prayuth was there at the same time, but there was no confirmation they met. After Gen Prayuth had left, nine vans with tinted windows were seen leaving, but it was not clear if Ms Yingluck was in one or where they were going.
An aide to a minister in the ousted government who also declined to be identified said some people, including his minister, had been detained. A former aide to Ms Yingluck said she been out of telephone contact for hours.
Ms Yingluck was forced to step down as prime minister by a court on May 7 but her caretaker government, buffeted by more than six months of protests against it, had remained nominally in power, even after the army declared martial law.
Gen Prayuth also summoned hundreds of civil servants and told them he needed their help.
"We must have economic, social and political reforms before elections. If the situation is peaceful, we are ready to return power to the people," he said.
The military has censored the media, dispersed rival protesters and imposed a nationwide 10pm to 5am curfew.
The armed forces have a long history of intervening in politics -there have been 18 previous successful or attempted coups since Thailand became a constitutional monarchy in 1932.
Bangkok was mostly calm and life appeared normal, but there was some opposition to the takeover. Several hundred people, including students, gathered in a central shopping district despite a ban on protests by five or more people.