Three Senegalese opposition lawmakers were arrested on Tuesday amid the fallout from parliament's move to delay a presidential vote by 10 months that prompted West Africa's economic and political bloc to call for the re-establishment of the electoral calendar.

Lawmakers late on Monday approved a last-minute amendment to hold the vote on December 15, instead of February 25, sealing an extension of President Macky Sall's mandate that has sparked street protests and international alarm.

The spokesperson for dissolved opposition party Pastef, El Malick Ndiaye, said via message that three lawmakers from the opposition coalition Yewwi Askan Wi had been arrested over the course of the Tuesday. A former police captain was also detained, he said.

One of those arrested, Guy Marius Sagna, was among the MPs who tried to physically block Monday's vote from happening in parliament by blocking the dais.

"Senegal has definitively sunk into dictatorship," Ndiaye said.

The police did not immediately respond to a request for comment.

The surprise postponement has dismayed those who thought Senegal would stick to a standard electoral course - something that has become increasingly uncommon in West Africa, where ECOWAS is grappling with the fallout from a string of military takeovers in other countries in recent years.

The US Department of State on Tuesday expressed deep concern with the postponement of the election, saying the move ran "contrary to Senegal’s strong democratic tradition."

The Economic Community of West African States (ECOWAS) did not refer directly to the new election date, but it suggested in a statement that the bloc views the postponement as unconstitutional.

"The ECOWAS Commission encourages the political class to urgently take the necessary measures to re-establish the electoral calendar in accordance with the provisions of the constitution," it said.

Sall, who is not standing in the vote and has reached the constitutional limit of two terms in power, has said the delay was necessary due to a dispute over the candidate list and alleged corruption within the constitutional body that handled the list.

'WE MUST FIGHT'

On Tuesday, ruling coalition lawmakers held a press conference to defend the move, urging citizens to take part in a national dialogue to ensure a free and transparent vote.

"We did what we needed to do and we will take responsibility for it," said Cheikh Seck, one of the MPs.

What happens next is unclear. Some opposition figures have launched legal challenges that could lead to prolonged wrangling in the courts.

The streets of Dakar were quiet on Tuesday with no immediate sign of further unrest that many have warned will be the inevitable consequence of the unprecedented postponement.

"We never thought that such a situation could happen. ... To decide like this overnight, it can only lead to confusion," driver Pape Sene said at an intersection that was bustling with usual day-time traffic.

"How I see the situation, no one will come out because people are discouraged," said another passerby, university student El Hadj Malick Diouf. "If I were them, I wouldn't give up. We must fight, once and for all."

Since Sall announced the delay in an address to the nation on Saturday, the authorities have cracked down on street protests in the capital, restricted mobile internet, and pulled a private TV channel off air.

"The postponement of Senegal’s presidential election puts the country on a dangerous path towards dictatorship, and must not be allowed to stand," said US Senate Foreign Relations Committee Chair Ben Cardin in a statement.