Voting was taking place behind closed doors, with the first clause winning the support of 373 lawmakers in the 550-member house, an AFP journalist said.

The MPs were debating the other clauses due to determine the overall outcome.

The bill, which had already led to deputies trading punches at an earlier stage, received initial support in a secret ballot on Tuesday.

In a related development, German Chancellor Angela Merkel's spokesman said she would raise concerns over the state of democracy in Turkey when she meets strongman Recep Tayyib Erdogan next week.

"For the domestic stability of every democracy, it is important for every relevant group in society to be also represented in parliament. The situation will certainly be raised by the chancellor on the sidelines of the humanitarian summit with Turkey's president," said Steffen Seibert.

Under current Turkish law, MPs have the right to full immunity from prosecution. If the new bill is approved, 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 members 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 has waged a three-decade insurgency in the southeast.

The PKK is considered a terrorist group by Ankara and its Western allies. Following the collapse of a two-year ceasefire in summer 2015, Turkey has been waging an offensive against the organisation.

- ' Turkey is secular '

The session on Friday opened with a group of opposition Republican People's Party (CHP) lawmakers walking out in protest against 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.

President Erdogan said on Friday the bill would be put to public vote if the support remained below 367.

"If it is not possible to get 367 (votes), and an outcome between 330-367 emerges, then this will go to public," Erdogan said, according to media reports.

"My esteemed nation will give the necessary answer."

The bill sparked violent scuffles in parliament this month with frustrated lawmakers exchanging fisticuffs and kicks.

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.