The Foreign Office is attempting to secure the release of a British teacher jailed in Sudan after children in her class named a teddy bear Muhammad.
Gillian Gibbons, 54, from Liverpool, received 15 days in jail for insulting religion; she will then be deported.
About 300 people protested against Ms Gibbons' actions in Khartoum after Friday prayers.
The Foreign Office was in contact with Sudan's government overnight and will repeat demands for her release.
The teacher's son John, a 25-year-old marketing consultant, told the Daily Mail the family are struggling to take in the punishment.
"It's really difficult at the moment, my head is everywhere," he said.
Mrs Gibbons' son added: "I don't want the verdict to lead to any anti feeling towards Muslims."
Prime Minister Gordon Brown has contacted Mrs Gibbons' family for a second time, speaking to a close relative of the teacher.
A Downing Street spokeswoman said: "We are doing all we can to ensure her release. Various intense activity is ongoing but I'm afraid it's not helpful for me to get into the detail of what that is at the moment."
Foreign Secretary David Miliband has expressed "in the strongest terms" the UK's concern at her detention.
The Sudanese ambassador, Omer Siddig, was called back to the Foreign Office to explain the decision.
Officials said that during his 45-minute meeting on Thursday Mr Miliband also spoke to the Sudanese acting foreign minister for 15 minutes on the telephone.
Meanwhile, Archbishop of Canterbury Rowan Williams said he could not "see any justification" for the sentence, calling it an "absurdly disproportionate response" to a "minor cultural faux pas".
The former UN Special Representative to Sudan, Jan Pronk, who was expelled from the country in 2006, said it was best not to protest against the sentence.
"The important thing is to get the lady out as soon as possible. The prison conditions are very harsh in Sudan."
About 300 people marched through the streets of Sudanese capital Khartoum in protest against Mrs Gibbons after finishing prayers on Friday.
Watched by police, the marchers walked through the streets waving their fists - one man waving a sword in the air - shouting "Allah is great".
Chris Ball, a friend and former colleague of Mrs Gibbons, told the BBC she had been "the victim of global political circumstances" and said the guilty verdict was "devastating", although "it could have been a lot worse".
In September, Mrs Gibbons allowed her class of primary school pupils to name the teddy bear Muhammad as part of a study of animals and their habitats.
The court heard that she was arrested on Sunday after another member of staff at Unity High School complained to the Ministry of Education.
The BBC's Adam Mynott, in Khartoum, said Mrs Gibbons apologised to the court for any offence she may have caused.
The school's director, Robert Boulos, told the AP news agency: "It's a very fair verdict, she could have had six months and lashes and a fine, and she only got 15 days and deportation."
Mrs Gibbons escaped conviction for inciting hatred and showing contempt for religious beliefs, and will now appeal.
He said Mrs Gibbons would only serve another 10 days in prison, having already spent five in custody since her arrest.
But Catherine Wolthuizen, chief executive of Fair Trials International, said 15 days in a Sudanese prison for an "innocent misunderstanding" is a "serious and harsh punishment indeed".
The prison Mrs Gibbons is expected to be held in is mainly used to house women convicted of making and selling alcohol.
It is reported to be overcrowded, with 1,200 inmates, and sometimes 20 women and their children sharing a single cell.