Egypt's prosecutor-general has ordered an official investigation into a number of opposition politicians who are boycotting next month's presidential election, as President Abdel Fattah el-Sisi looks set to extend his term.

Nabil Sadeq, Egypt's prosecutor general, said in a statement on Monday that 13 individuals may be summoned to the Giza office for "incitement against the state" and attempting to "overthrow the regime".

This comes as opposition parties called for a boycott of the March vote last month.

Among those to be investigated is Hamdeen Sabahi, Sisi's only rival in the 2014 presidential elections.

Experts say Sisi, who is seeking his second term in office, is almost guaranteed to be re-elected after he made it nearly impossible for any real political opponent to challenge his firm grip on power.

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What happened to the hopefuls?

Several potential candidates have either been arrested or faced threats, intimidation and physical violence, forcing them to drop out.

The former head of Egypt's armed forces, Sami Anan, suspended his run for the country's presidency, hours after he was arrested by the army on accusations of committing violations that "warrant official investigation".

Former Prime Minister Ahmed Shafik's plan to run was short-lived after he withdrew his potential candidacy.

"I saw that I would not be the ideal person to lead the state during the coming period," Shafik said in a statement posted on Twitter.

One of Shafik's lawyers accused the Egyptian government of putting pressure on the 76-year-old by threatening to re-investigate previous corruption allegations against him, the New York Times reported.

Meanwhile, Khaled Ali, an opposition leader who ran in the 2012 presidential polls, quit the race after stating his intention to run.

"The opportunity for hope in this presidential election has gone," the human rights lawyer told his supporters.

Ali was sentenced to three months in prison last year for "offending public decency" after he allegedly made an obscene gesture during a protest against Egypt's decision to cede control over two islands to Saudi Arabia.

In December 2017, Ahmed Konsowa, an army colonel, was sentenced to six years in prison after announcing his candidacy.

Who are the candidates?

Sisi, 69, announced his presidential bid in Cairo last month, where he stressed the importance of citizens voting to "preserve the democratic experience that began four years ago".

"Your participation in the election will be a strong message," he said, before jokingly adding that Egyptians would "become exhausted with me again because Egypt needs every sacrifice".

The former Egyptian army chief is pitted against Moussa Mostafa Moussa, chairman of the liberal El-Ghad Party.

Moussa, a Sisi loyalist, submitted his candidacy documents 15 minutes before the deadline.

"I will not be a background actor [for Sisi in the election]," he said. "I am looking forward to a strong and real electoral competition with the president, especially as I have a strong platform that relies on young people and addresses the rising prices."

However the 66-year-old has repeatedly endorsed Sisi, and last year formed a campaign called, "Supporters of President el-Sisi's nomination for a second term".

Egyptians took to social media and used the hashtag Al-Kombares, which loosely translates to someone playing the role of an "extra", to mock Mousa's candidacy and the upcoming poll.

A runoff vote will be held in April if no candidate receives more than 50 percent support in the first round.

"I think the word 'election' is probably too generous," Timothy Kaldas, non-resident fellow at the Tahrir Institute for Middle East Policy, told Al Jazeera.

"The question is: what's worse, a convincing sham, or one that's transparently a sham?"