ISTANBUL (AP) — Turkey’s prime minister says opposition parties have the right to file objections to the outcome of a recent referendum on expanding presidential powers, but warned that calling for street protests was unacceptable.

Binali Yildirim said Wednesday the electoral board would rule on the main opposition Republican People’s Party’s request for the referendum’s annulment. Opposition parties have complained of a series of irregularities, particularly an electoral board decision to accept ballots without official stamps, as required by Turkish law.

Yildirim said the “the path to seek rights” should be limited to legal objections and urged the opposition to accept the vote’s outcome.

Thousands have protested in Istanbul and Ankara since Sunday’s referendum, which has set into motion the transformation of Turkey’s system of government from a parliamentary into a presidential one.