Egypt's presidential election was extended to a third day on Tuesday night, in the latest of a series of attempts to encourage more people to vote.

The announcement followed a last-minute decision to turn Tuesday into an impromptu public holiday – the first sign that officials were concerned about low turnout.

Egypt's interim president, Adly Mansour, has said the state is neutral in the race. But critics portrayed the moves as an attempt to boost the credibility of the former army chief Abdel Fatah al-Sisi, who is expected to win the poll easily. A low turnout would undermine the argument often made by Sisi's backers that he has the backing of an overwhelming majority of Egyptians.

"It's like they're trying to force people to go vote," said Ahmad Abdallah, a spokesman for the 6 April movement, a secular campaign group that is boycotting the elections. He argued that the extension of the voting period showed that most voters were indifferent to Sisi. "The Egyptian people have realised that whoever comes to power by tanks will not leave through elections – so they are boycotting it."

Egypt's last presidential run-off, in 2012, had a turnout of 51.86%, and Sisi had hoped for far greater participation this time. Sisi said several times that he wanted a turnout of 40 million to show that "there is a consensus on a national level".

But even those loyal to Sisi admitted that turnout was substantially lower. The owners of Cairo's main shopping mall, City Stars, closed it early on Tuesday to encourage shoppers to vote, and well-known pro-Sisi TV hosts implored viewers to participate.

"Where are the people?" asked Lamis al-Hadidi, a popular talkshow host. "We have to go down tomorrow. Just for tomorrow. Go down for just an hour. Half an hour." Hadidi's husband, Amr Adeeb, himself a prominent television personality, said: "I'm willing to cut my veins for the country – right now, on air – for people to go down and vote."

The decision to extend the vote had administrative and political implications for international observers assessing the election. The EU and the Global Network for Rights and Development said they would continue to observe the election as normal, but other groups said they were still deliberating.