* Former PM Yingluck was detained by military on Friday
* Yingluck's brother Thaksin is at heart of political crisis
* King acknowledges military takeover - army
* Pro-Thaksin supporters, democracy activists defy junta (Adds U.S. Pentagon cancels training, visits)
By Panarat Thepgumpanat and Paul Mooney
BANGKOK, May 24 (Reuters) - Former Thai Prime Minister Yingluck Shinawatra was in a "safe place" on Saturday after being held by the army following a coup, an aide said, as opposition to the takeover grew among her supporters and pro-democracy activists.
The army moved on Thursday after failing to forge a compromise in a power struggle between Yingluck's populist government and the royalist establishment, which brought months of sometimes violent unrest to Bangkok's streets.
Consolidating its grip, the military dissolved the Senate on Saturday, the only legislative assembly still functioning in Thailand. It also sacked three senior security officials who were seen as close to the ousted government.
The military detained Yingluck on Friday when she and about 150 other people, most of them political associates, were summoned to an army facility in Bangkok.
More people were summoned over the weekend, including some outspoken academics and journalists. The bosses of 18 newspapers and private and public sector economic administrators were also called to meetings with the military.
A senior officer told Reuters Yingluck could be held for up to a week and media reported she had been taken to an army base in Saraburi province north of Bangkok, but an aide denied that.
"Now she's in a safe place ... She has not been detained in any military camp. That's all I can say at this moment," said the aide, who declined to be identified.
Thailand's political woes are the latest chapter in a nearly decade-long clash between the Bangkok-based establishment and Thaksin Shinawatra, a former telecommunications tycoon who broke the mould of Thai politics with pro-poor policies that won him huge support and repeated electoral victories.
Thaksin was ousted in a 2006 coup and left the country after a 2008 graft conviction, but he remains Thailand's most influential politician and was the guiding hand behind the government of Yingluck, his sister.
The military also detained Thaksin's adult son, and Yingluck's nephew, Panthongtae Shinawatra, according to posts on social media, but his sister later said that was not true.
Army deputy spokesman Winthai Suvaree told a news conference that anyone being held would not be detained for more than seven days. He did not mention Yingluck.
The military also issued an order to financial institutions to freeze dealings with two former ministers in Yingluck's cabinet who had not responded to a military summons.
The army also said King Bhumibol Adulyadej had acknowledged the military takeover, a significant formality in a country where the monarchy is the most important institution.
An undercurrent of the crisis is anxiety over the issue of royal succession. The king, the world's longest-reigning monarch, is 86 and spent the years from 2009 to 2013 in hospital.
Crown Prince Vajiralongkorn does not command the same devotion as his father, but some Thaksin supporters have recently been making a point of their loyalty to the prince.
"REFORMS BEFORE ELECTION"
Despite international calls for the restoration of democratic government, army chief General Prayuth Chan-ocha has not promised a swift return to civilian rule, insisting there must be broad reforms and stability first.
"We must have economic, social and political reforms before elections," Prayuth told civil servants on Friday in his first comments on his plans since the coup. "If the situation is peaceful, we are ready to return power to the people."
But reforms could take many months and stability could be elusive.
Many countries have issued travel warnings for Thailand.
The United States swiftly condemned the coup and the State Department suspended about $3.5 million in military aid.
The Pentagon said on Saturday it was cancelling training and readiness exercises with Thailand, as well as a visit to Thailand by U.S. Pacific Fleet Commander Admiral Harry Harris, and one by a top Thai commander to the U.S. Pacific Command.
Human Rights Watch said rights in Thailand were in "free fall". But in what appeared to be a quick move to win over some of Thaksin's core supporters, Prayuth said on Friday paying farmers money owed under a failed subsidy scheme organised by Yingluck's government was a priority.
STIRRINGS OF OPPOSITION
The military has banned gatherings of more than five people, censored the media and imposed a 10 p.m to 5 a.m. curfew, but that has not stopped some from showing their disapproval.
About 200 people gathered outside a mall complex in north Bangkok early on Saturday, holding up handwritten slogans such as "Anti the Coup" and "Get out Dictators".
Police tried to move them on, a Reuters reporter said. The crowd then moved to the Victory Monument but police tried to block them. There was some pushing and plastic water bottles were thrown.
About 100 people gathered in a nearby shopping area before soldiers dispersed them, detaining several, a Reuters photographer said.
About 200 people gathered for a second day in Chiang Mai, Thaksin's hometown, and soldiers detained at least six people, a Reuters reporter said.
Such small protests appear spontaneous and leaderless but the real danger for the military would be a sustained mass campaign by Thaksin's "red shirt" loyalists.
Thaksin has not commented publicly since the coup.
The use of force to put down protesters could squander any legitimacy the military leaders may have after saying they took power in the first place to end violence and restore order.
A 2010 crackdown on Thaksin's supporters ended in serious bloodshed and damage to the army's image. Just over a year later a pro-Thaksin government was back in power after Yingluck's sweeping election victory. (Writing by Robert Birsel and Alan Raybould; Additional reporting by Khettiya Jittapong; Editing by Angus MacSwan)