The embattled government has been cracking down on its critics
A member of the Malaysian cabinet and long-standing member of parliament, Zaid Ibrahim, has resigned.
He quit one day before opposition leader Anwar Ibrahim said he planned to try to topple the government.
Mr Zaid was resigning because of his government's use of the controversial Internal Security Act, an aide said.
The ISA allows for indefinite detention without trial and was used to detain a member of parliament, a blogger and a journalist over the weekend.
Mr Zaid, cabinet minister responsible for legal affairs, was brought into cabinet in the Barisan Nasional (National Front) government of Prime Minister Abdullah Badawi in March.
Correspondents say that government is now under intense political pressure.
Led by Mr Anwar, a former deputy prime minister, the opposition plans to hold a large rally later on Monday and says it has secured police permission to do so.
Following elections in March, in which Mr Anwar's forces made significant gains at the expense of the government, Mr Anwar has claimed he needs only 30 defections from the government to take power.
Those plans may be delayed, he has admitted, not least because the government sent about 50 of its MPs on a trip to Taiwan late last week.
His opposition alliance has 82 MPs in the 222-strong parliament.
Mr Anwar's aides told reporters he planned to deliver a letter to the prime minister on Monday seeking a meeting with him for a "peaceful power transition" to the opposition Pakatan Rakyat alliance.
Mr Abdullah had agreed to hand over power in 2010 to his deputy Najib Razak, but has come under pressure from top leaders in his United Malays National Organisation (UMNO) to do so earlier. UMNO is the main party in the Barisan Nasional coalition.
Mr Zaid's resignation came after several protests by members of the government at its use of the ISA, in what correspondents said was a sign of growing division within the ruling party.
The ISA is a legacy of British colonial rule and has been used to imprison opponents of the government for years, including people alleged to be participants in the US-led "war on terror".
Detentions are usually for an initial two years but can be rolled over at the government's discretion, with no requirement for a trial or for charges to be laid.
"There were ample punitive laws to act against lawbreakers without having to invoke the ISA. [The law] should only be used on armed terrorists or those out to topple the government by force," Mr Zaid had said late on Sunday, before his resignation.
He said the arrest of Tan Hoon Cheng from the Chinese-language daily Sin Chew for reporting on racially charged comments by a Malay Muslim ruling party politician gave the perception of "selective prosecution" as no action was taken against the politician. Ms Tan has since been freed.
"The government must change but reform cannot take place if it continues with the old thinking and old philosophies," Mr Zaid had said.
"I joined [Mr Abdullah's] administration to make changes and everybody knows, I am also for a more open form of press reporting," he told local reporters.
Call for review
Separately, Bernard Dompok, a minister in the Prime Minister's Department, was reported to have said on Sunday that the arrests had put the government in a bad light.
The transport and health ministers reportedly called for a review of the law.
The Home Minister Syed Hamid Albar defended the government's actions, saying it was necessary to prevent racial tensions flaring up.
Blogger Raja Petra Kamaruddin was jailed apparently for his political commentary, and has been charged with sedition and defamation, while opposition Democratic Action Party politician Teresa Kok was accused of objecting to a mosque broadcasting its prayers too loudly.
Prime Minister Abdullah's coalition has been in power since independence in 1957.