The broadcast, Election studio, the final of a set of five pre-election 75-minute debate shows run by ERR, featured Mr Ratas, plus leaders or prime ministerial candidates of the other major political parties contesting Sunday's election.

Mr Ratas' was reportedly phoned about a dozen times during the course of the broadcast, by Reform Party youth members, something which his office says were attempts to troll him.

The callers may have been irked by the fact that Mr Ratas, as seen in a photo gallery of the broadcast on news portal Delfi, had his phone with him in the first place, which they might have seen as giving him an unfair advantage.

Reform's youth wing has, however, exhibited contrition for its members' antics.

"On behalf of the Reform Youth, I wish to apologize for our behavior last night during the debate of prime minister candidates,'' said Kristo Enn Vaga, secretary general of Reform's youth branch, on Sunday.

''It was a juvenile prank; we simply overheated amidst all the election buzz. We wanted to draw attention to the fact that Prime Minister Jüri Ratas often reads out responses written by others from his phone, during live programs and Riigikogu question time. We had the impression that the same was heppening during yesterday's debate as well," Mr Vaga continued.

''We thought that it would be fair to other participants of the debate if we did not let the prime minister use external help in the debate [ie. via his mobile phone-ed.] ... This definitely does not justify our behaviour and we definitely did not wish to concern anyone," he added.

The Reform Party and Centre have overall been neck-and-neck in most recent opinion polls.

Small number of other violations

Meanwhile the Police and Border Guard Board (PPA) has reported 10 calls to investigate potential election violations, on Sunday.

These potential violations include the placing of outdoor election advertisements, banned since late January under electoral law. These may simply be perceptions rather than actual violations.

"For example, people perceive an image on a vehicle or trailer as election advertising, or a party's leaflets have been left on a table at a shopping centre where voting is also taking place,'' said Marko Ild, leading law enforcement official at the PPA.

''We have also been notified about concerns regarding social media advertising. We will check and evaluate all such messages and, if necessary, intervene," Ild said. Social media and TV advertising is not covered by the pre-election ban.

As at 12.00 EET on Sunday, just under 47% of the electorate had already cast its vote. This compares with just over 42% at the same stage in the 2015 general election. Highest voter turnout was in Harju County, the most populous electoral district, at 52.6%. The lowest turnout as at midday was in the easternmost electoral district of Ida-Viru County, where just over 30% of those eligible had voted.