The local elections, covering over 150 local authorities, spanning metropolitan and district councils, unitary authorities and London boroughs, will prove a crucial test for her Conservative party-led government as the first reflection of public opinion since the snap general election in June past year.
The Conservatives lost Trafford Council after 14 years in power.
Despite retaining overall control, the Conservatives lost individual seats in Westminster and Wandsworth.
The majority of councils will declare votes overnight after polling closes at 10 PM (local time), but some will release results during the day on Friday.
In Southampton Mr Letts said he was "immensely disappointed" to have lost his seat after 18 years on the council.
All the council seats in London, Birmingham, Manchester, Leeds and Newcastle are up for election.
Labour's council leader in Southampton lost his seat to the Conservatives, but the opposition party's own leader was also voted out.
"We've got a lot of new members and we've brought the party together".
The Lib Dems have gained at least 20 seats across the country.
The results from Great Yarmouth and Norwich are expected later today.
Most of the seats being contested were last up for grabs 2014, when Labour made big gains.
All parties will be hoping to pick up seats from UKIP.
Meanwhile, social media reports suggested residents in areas piloting the UK Government's controversial ID trials to reduce voter fraud had been barred from voting. She tweeted: 'Just been round the C Palace polling stations. Was informed that two people had already turned up without ID this morning so had been unable to vote.
Other changes saw Christian Hogg (Liberal Democrat) win Fletton and Stanground from Labour and Shazia Bashir (Conservative) take North from Labour.
The prime minister's spokesman said the "vast majority" of people had voted without a problem.