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Sri Lanka's president said Sunday that the main reason he decided to sack his prime minister was the alleged involvement of a Cabinet minister in a plot to assassinate him.

In a televised address to the nation, President Maithripala Sirisena said a person questioned by investigators had revealed the name of a minister in an alleged plot to assassinate him and a former defense secretary.

He said the only choice for him under the circumstances was to dismiss Ranil Wickremesinghe and invite his former nemesis and ex-strongman Mahinda Rajapaksa to take over as prime minister and form a new government.

"This information (received by investigators) contains a number of details hitherto hidden to the people," Sirisena said. "The informant has made a statement regarding a Cabinet minister involved in the conspiracy to assassinate me."

He did not reveal the name of the minister.

Even though Sirisena's supporters had been talking about an alleged plot to assassinate him for weeks, Sunday was the first time Sirisena had made a public statement on it.

Meanwhile, one person died and two others were wounded on Sunday in a shooting at the Petroleum Ministry, in the first violent incident since the political turmoil began on Friday with the sacking of Wickremesinghe.

Pushpa Soyza, a spokeswoman at Colombo National Hospital, said three people were admitted to the hospital following the shooting, and one of them had died.

Arjuna Ranatunga, who was petroleum minister under Wickremesinghe, said one of his security guards opened fire when Rajapaksa supporters mobbed him and protested against him entering the ministry premises.

Wickremesinghe has called Sirisena's move to sack him unconstitutional and said he can prove his majority support in Parliament.

On Saturday, Sirisena suspended Parliament in an apparent move to give Rajapaksa time to try to muster enough support to survive any no-confidence vote.

The speaker of Parliament urged Sirisena to safeguard Wickremesinghe's rights.

Speaker Karu Jayasuriya said in a letter to Sirisena on Sunday that the continued suspension of Parliament would have "serious and undesirable consequences."

Opposition lawmakers, supporting the new prime minister, asked Wickremesinghe to vacate his official residence or face a forcible eviction.

Hundreds of Wickremesinghe supporters continued to gather outside his official home on Sunday for the second consecutive day, waving party flags and denouncing Sirisena and Rajapaksa. Buddhist monks performed religious rites to invoke blessings on Wickremesinghe.

Jayasuriya said in the letter that he received "a request to protect the rights and privileges" of Wickremesinghe "until any other person emerges from within Parliament as having secured the confidence of Parliament." He said the request came from two senior lawmakers from the sacked prime minister's party.

"This request is especially important in the context where various persons are reported to have issued threats via the media," Jayasuriya said, adding that "the forcible takeovers" would have "serious international implications."

Tensions have been building between Sirisena and Wickremesinghe for some time, as the president did not approve of some of the economic reforms being introduced by the prime minister. Sirisena was also critical of investigations into military personnel accused of human rights violations during Sri Lanka's long civil war, which ended in 2009.