A woman died in the small hours of Monday in a Sofia sports hall where ballot papers from electoral committees were being processed, reports suggest.

The woman was taken to hospital but died later.

Media reports differ as to what caused the incident. The Central Electoral Commission (CEC) has said she is unable to confirm and has called for patience amid ballot processing difficulties, as local elections were held alongside a national referendum on Sunday.

Daily Trud also says several people "fainted" during the night as tensions grew in the hall, but the CEC has not confirmed.

Members of regional election commissions (the "subdivisions" of municipal election authorities in Sofia) were not allowed to leave the room where ballots were being counted, a deputy chair of one of the commissions told the Bulgarian National Television.

An official carrying ballot papers from his polling station told private national NOVA TV station ballot processing after the Sunday vote was "slow", and the Arena Armeec hall, where municipal papers were being handed after the vote, was packed with people.

He reported there had been failures in the organization, with many people being unable to reach their homes after reaching the hall and handing out their ballots.

In his words, as of 07:30 (EET) on Monday there were 5000 people in the hall, with just 400 vote protocols processed, out of 1500.