Turkey's pariament has completed a first round of voting on a bill that will strip some MPs of immunity from prosecution - a move members of the pro-Kurdish Peoples' Democratic Party (HDP) say targets them.

The bill, championed by the ruling Justice and Development Party (AKP), would amend the constitution, adding a temporary clause to remove immunity from a total of 138 deputies in the parliament - the number who are currently facing criminal investigations.

Late on Tuesday, the parliament voted to send the bill forward for debate with 348 votes in favour, 155 against and eight abstentions.

On Wednesday, its first article was approved with 350 votes for and 148 against, and its second article approved 357 to 149.

The second of two voting rounds will be held on May 20.

The move needs the support of at least 367 deputies in the 550-seat assembly to be directly passed. It would go to a referendum if it fell short of that number but reached 330 votes.

President Recep Tayyip Erdogan has the authority, though, to take the bill back to parliament for another vote, rather than to hold a referendum.

Currently, 51 opposition Republican People's Party (CHP) MPs, 50 HDP MPs, 27 AKP MPs, nine Nationalist Movement Party (MHP) MPs and one independent are facing investigations.

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Combined, there a total of 667 proceedings against parliamentarians. Some 405 of them are against HDP MPs.

"Someone who is aiding and abetting terrorism cannot be an MP," MHP's Mehmet Parsak said, speaking in favour of the bill before the vote. "The mentality that supports terrorism can not seek harbour in the parliament."

Both the MHP and the CHP support the bill in what some analysts see as a move aimed at winning support from nationalist voters who want to see Kurdish parliamentarians prosecuted.

"If they manage to eliminate HDP from politics and the parliament, it is clear to us that CHP and the rest of the opposition will be the next in line," the HDP's Idris Baluken told parliament.

Voting on the bill will be held by secret ballot and several MPs have said they may not vote with their parties.

'Completely constitutional'

Erdogan has previously called for members of the HDP to face prosecution, accusing them of being the political wing of the outlawed Kurdistan Workers Party (PKK).

The HDP rejects that accusation and says Erdogan is trying to push them out of the parliament so that he can alter the constitution and replace the current parliamentary system with a presidential one.

Erdogan denies that charge.

In an open letter to the European Parliament the HDP criticised the bill as unconstitutional.

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"If passed, this motion would suspend Article 83 of the Constitution, which guarantees parliamentary immunity, through addition of a provisional clause," Selahattin Demirtas, a co-leader of the HDP, wrote.

"Lifting parliamentary immunity with such an anti-constitutional move would extend the Erdogan-AKP bloc’s monopolistic grip on the legislative body."

Huseyin Ozcan, who teaches constitutional law at Istanbul University, told Al Jazeera he believed the proposed bill was "completely constitutional.

"Article 83 of the Turkish constitution makes it clear that the parliament has the right to remove MPs' immunity under special circumstances, and MPs decided we are now facing special circumstances," he said.

As a result of a similar move in 1994, several pro-Kurdish MPs were stripped of immunity.