Sri Lankan governing party members of parliament have begun moves to impeach Chief Justice Shirani Bandaranayake.
A government spokesman said the chief justice's conduct had "affected the sovereignty of the people", without elaborating.
An opposition MP said the move was evidence that Sri Lanka was becoming a "constitutional dictatorship".
Separately, the US has said that "interference with the judicial process" in Sri Lanka should stop.
It said "serious human rights violations" including disappearances, torture, summary killings and threats to free expression persisted in Sri Lanka, despite the end to the bloody separatist war with Tamil Tiger rebels in 2009.
The comments came at the Universal Periodic Review at the UN Human Rights Council in Geneva - a peer assessment which each country goes through roughly every four years.
Ms Bandaranayake was appointed Sri Lanka's first woman chief justice last year.
A parliamentary motion to impeach her was submitted on Thursday, officials say. Impeachment is the only constitutional way to remove a sitting chief justice.
President Mahinda Rajapaksa's UPFA party has enough seats for the impeachment to succeed, but the process could take months.
The initial proposal has received the approval of the required number of lawmakers, with nearly 120 MPs from the government side signing the motion.
The government has declined to give full details of why it is moving against Ms Bandaranayake.
But government spokesman Keheliya Rambukwella said the chief justice's behaviour and conduct in the past year had "affected the sovereignty of the people", without giving further details.
An opposition MP, Eran Wickramaratne, said the move was part of a trend in which the country was becoming a "constitutional dictatorship".
Correspondents say this is the latest example of the growing tensions between the government and the judiciary.
Recently, Sri Lanka's Supreme Court ruled that a bill proposing the transfer of vital powers held by Sri Lanka's provinces back to the central government needed prior approval from provincial councils.
On Tuesday, the Human Rights Institute of the International Bar Association (IBAHRI) told the BBC's Sinhala service that the government's move to impeach the chief justice risked being seen as an attempt to curtail the independence of the judiciary.
The government has categorically rejected such suggestions, describing them as "unfortunate".
At the Universal Periodic Review later on Thursday, many countries - including Sri Lanka's close friends, Cuba, Iran and Pakistan - praised the country's post-war rights record.
But the US and other Western countries were strongly critical.
Critics say the country has done too little to address allegations that the army committed war crimes, and accuse the government of perpetrating torture and enforced disappearances in peace time.
They dispute Sri Lanka's assertion that the role of the army has been scaled back in the former theatre of war in the north, and want UN member states to subject the country's government to tough scrutiny.
Sri Lanka's government denies the accusations of rights groups, saying on Thursday that fewer than 8,000 people were killed in the final months of the 26-year war, and arguing that it deserves more credit for putting thousands of former Tamil Tigers through a rehabilitation process.
It said alleged war crimes - a subject of regular international censure - were being looked into by a special military court.
But its delegates gave inconsistent figures for the number of court hearings so far, and even for the number of officers that sit in it.
Three randomly-selected members of the council - India, Spain and Benin - will now write the peer review of the Sri Lankan rights situation which will be presented to the full Human Rights Council for approval.