The bill, which had already led to deputies exchanging blows at the committee stage, received initial support in a secret ballot in the 550-seat parliament on Tuesday.

Voting on Friday was taking place behind closed doors, with results expected to be made public later in the day.

Under current Turkish law, members of parliament have the right to full immunity from prosecution. If the new bill passes, it would lift the immunity of roughly 138 deputies from all parties who face potential prosecution.

But the pro-Kurdish Peoples' Democratic Party (HDP) says the bill is essentially a move to expel its MPs from parliament.

HDP lawmakers are particularly vulnerable to prosecution on allegations of links or even verbal support for the outlawed Kurdistan Workers Party (PKK), which is fighting a renewed insurgency against the Turkish state.

The session on Friday opened with a group of opposition Republican People's Party (CHP) lawmakers walking out of the room in protest of parliament speaker Ismail Kahraman, who stirred controversy in April when he proposed a religious constitution.

"We don't think it is appropriate for you to run this session," the secular CHP party representative Levent Gok said, quoted by Turkish media.

Before leaving the parliament floor ahead of the secret ballot, MPs chanted: "Turkey is secular and will remain so."

The ruling Justice and Development Party (AKP) needs to win 367 votes in parliament -- a two-thirds majority -- to push the legislation through directly at Friday's second-vote stage.

A three-fifths majority -- 330 votes -- would be enough to call a referendum on the issue.

The bill sparked violent scuffles in parliament this month with frustrated lawmakers exchanging blows with their fists and even feet rather than discussing the document.

The HDP, the third largest party group in parliament, has said the bill could lead to the prosecution of 50 HDP deputies out of its total contingent of 59.