Myanmar's influential parliamentary Speaker Thura Shwe Mann, 68 - a former general and contender for the presidency - has been removed from the leadership of the ruling Union Solidarity and Development Party (USDP).
The purge reflects infighting among the military elite, analysts say, and leaves President Thein Sein in a position of strength with USDP now stacked with his allies.
In a dramatic move, armed police descended on the headquarters of the party in Naypyitaw late on Wednesday night and sealed it off.
Mr Shwe Mann was at home and guards were placed at his house.
His deputy, party secretary general Maung Maung Thein, told Reuters he had received a phone call from someone telling him he need not come to the office any more.
The deputy head of the party, U Htay Oo, a former minister of agriculture, and also a former general and Thein Sein loyalist, has taken over as chairman.
Mr Tin Naing Thein, a minister in the President's office, has taken over from Mr Maung Maung Thein.
A well-placed source in Myanmar told The Straits Times: "This is an internal party coup.
"Thura Shwe Mann remains Speaker of Parliament but no longer leader of the party."
The swift purge had a long genesis in the growing tension between Mr Shwe Mann and President Thein Sein, in which the President appears to have checkmated the ambitious Speaker.
Mr Zaw Htay, a director in the President's office, told The Straits Times police had been deployed to maintain security. "No one will be arrested," he said, countering rumours that flew through the country on Thursday morning that Mr Shwe Mann was under house arrest.
"The government wanted to maintain a stable situation amid the reshuffle in the leadership at the party," Mr Zaw Htay said.
The reshuffle would not affect the upcoming Nov 8 general election, he added, saying that "the government will try to transfer power smoothly".
Minister of Information Yeh Htut, in an interview with Voice of America, hinted at the reasons for the purge.
He said: "Any party will have to go about changes when its leadership deviates from the party's policies, ignores the party members' will, prioritises personal profit over the country's interests and creates factionalism within the party.''
A Yangon-based analyst who asked not to be named, told The Straits Times that "Shwe Mann went too far, going against the government and the army.
"His personal rivalry against the President went too far. In the process, he also attacked the army."
Observers have pointed to several instances of friction between the two men.
•In what was seen as a snub to President Thein Sein, two of his key ministers, Mr Soe Thane and Mr Aung Min, had their applications to run for the USDP in constituencies of their choice turned down by the party.
•Mr Shwe Mann also turned down dozens of retired army officers who wanted to run under the party banner in the coming Nov 8 general election.
• In June, the Speaker allowed a vote in Parliament on a proposal that would have rolled back some of the military's powers.
The powerful military used its 25 per cent of reserved seats in Parliament to defeat the proposal, but the army leadership was upset that it had been put to a vote, insiders said.
• Mr Shwe Mann had also evolved a close working relationship with opposition leader Aung San Suu Kyi, even as her own ties with President Thein Sein, which had started warmly in 2011, cooled. Ms Suu Kyi's National League for Democracy is likely to emerge the single biggest party from the elections, but she is barred from the presidency because her two sons are British citizens. As a result, she would have to back someone else for the post - potentially even from outside the party. Political pundits had pointed to Mr Shwe Mann as a possibility. While his prospects for the presidency are now dimmer, that, however, still cannot be ruled out.
But President Thein Sein, although 70 and in frail health, remains in a strong position.
He has not explicitly said he is seeking the office again, but he has indicated he would be available for the position if asked.
Much depends on post-election bargaining, and while he is not contesting the election itself, under Myanmar's electoral college system, he could be voted president by sitting MPs.
Some analysts believe he could also be keeping the position safe for current army commander-in-chief, Senior General Min Aung Hlaing, who is due to retire this year.