Turkey's election authority on Wednesday rejected opposition requests to cancel a referendum that boosted President Recep Tayyip Erdogan's authority as police detained activists over street protests following the contested poll.
The narrow victory of the 'Yes' campaign in Sunday's referendum handed Erdogan sweeping new powers -- most of which will come into force after 2019 -- but was bitterly disputed by his rivals.
The controversy has stoked further political tensions over the rule of Erdogan, who has dominated Turkey since becoming premier in 2003 then president in 2014.
The main opposition Republican People's Party (CHP) and the pro-Kurdish Peoples' Democratic Party (HDP) had on Tuesday asked that the poll be scrapped over alleged violations.
Although the 'Yes' camp won with 51.41 percent, it was a narrower-than-expected victory with the opposition claiming the outcome would have been reversed in a fair poll.
Ten members of the Supreme Election Board (YSK) decided against annulling the vote, while only one voted in favour, the board said in a statement.
To the dismay of opposition parties and 'No' supporters, the YSK made a last-minute decision on Sunday to accept ballot papers in envelopes without an official stamp.
International observers from the OSCE and the Council of Europe rights watchdog denounced the move, saying it "removed an important safeguard".
Bulent Tezcan, CHP deputy leader, told CNN-Turk the YSK's decision to reject the petitions sparked a "serious legitimacy crisis."
- 16 protesters arrested -
There have been daily street protests in anti-Erdogan neighbourhoods in Istanbul since Sunday's referendum, with thousands chanting slogans and banging pots and pans in an angry show of discontent.
Istanbul police on Wednesday detained 16 leftist activists involved in demonstrations.
Among them was Mesut Gecgel, the Istanbul chairman of the Freedom and Solidarity Party (ODP), a leftwing movement not represented in parliament.
The party said he was detained on accusations of "agitating the public" by claiming the 'Yes' vote was illegitimate.
Gecgel's lawyer Deniz Demirdogen told AFP that anti-terror police raided the suspects' homes before dawn, saying 16 people were detained so far but arrest warrants had been issued for 38.
Describing the accusations as "strange", he added: "They are accused of provoking people to question the legitimacy of the 'Yes' in the referendum.
"But there's no such crime definition in the penal code."
Speaking in Ankara earlier, Prime Minister Binali Yildirim said the matter of the election result was closed and warned against further protests.
"Turkey is a state of law... and there can be no talk of anarchy, activities in the street," he said.
"I call on people not to give in to provocations or get caught up in incitement."
- 'All legal paths' -
CHP spokeswoman Selin Sayek Boke vowed that the party would use all means to challenge the result and called for a re-run of the vote.
"We will use all the legal paths and all legitimate democratic rights... No-one should doubt this," she told reporters in Ankara, saying such steps could even include "withdrawing from parliament".
But Levent Gok, CHP's parliamentary group leader, later rejected the idea of boycotting parliament, saying it would use "all legitimate means" to protect the parliament.
Turkey's leading satirical weekly Penguen meanwhile took a light-hearted but cutting look at the situation by printing a blank cover.
"From now on, covers without cartoons will also be counted as valid," said the caption, in reference to the controversy over the counting of unstamped ballots.
- 'Trump makes us happy' -
The referendum has also caused new friction in Turkey's relationship with the European Union, which it has long sought to join but which gave the 'Yes' victory the most lukewarm welcome.
By contrast, Donald Trump joined Russian President Vladimir Putin in congratulating Erdogan, with the Turkish strongman expressing optimism over his relationship with the new US leader.
Foreign Minister Mevlut Cavusoglu told reporters in Ankara that Trump and Erdogan would meet in May before a NATO summit.
"The way president Trump is approaching these matters makes us happy," Erdogan told CNN in an interview.