Around 40 protesters have gathered in Blaenau Gwent when the Labour Party chooses a new parliamentary candidate from an all-women shortlist later.
More than 40 protesters picketed the shortlist meeting last month
Local constituency party officers have boycotted the selection process in protest at the bar on male candidates.
The current MP, Llew Smith, is standing down at the next general election and Labour leaders have insisted on a female successor following a policy decision at national conference.
Of the 34 Labour MPs in Wales, just four are women and the party wants to increase numbers.
The three constituencies where current Labour MPs in Wales are retiring at the next election - Blaenau Gwent, Swansea East and Llanelli - have been told they must choose a female candidate.
But angered at the decision, party officials in Blaenau Gwent opted out of the selection process, which is being run by Welsh Labour centrally.
The decision-making meeting is taking place on Thursday evening with the winning candidate selected by one-member, one-vote ballot of local party members.
Last month, activists protested outside the shortlist meeting.
Peter Law: Local party forced into an 'experiment'
Five women are in the running for the candidacy, with the favourite being London-based union official Maggie Jones.
The other four contenders are Sonia Behr, Sian James, Gwennan Jeremiah and Mary Creahg.
Peter Law - the constituency's assembly member - was expected to be among the protesters.
He has said that the local party is being forced to take part in an experiment.