The Queen will welcome Mexican president Enrique Pena Nieto to the UK as campaigners urge politicians to tackle the country over its human rights record.
The president and his wife Angelica Rivera - a former soap opera actress - are staying at Buckingham Palace during the three-day state visit.
Mr Pena Nieto arrives in London today, but will receive his official ceremonial welcome on Horse Guards Parade tomorrow, followed by a state banquet at the Palace in the evening and talks at Downing Street with Prime Minister David Cameron on Wednesday.
Protest group Yo Soy 132 Londres has written to the Queen raising concerns and is planning demonstrations during his trip, while Amnesty International UK will deliver a 13,000-strong petition to the Mexican embassy demanding that the president deals with the country's "torture crisis".
Mr Pena Nieto and his administration have faced widespread public anger over the handling of the notorious abduction and apparent murder of 43 trainee teachers by corrupt police in league with gang members.
The students' disappearance in Ayotzinapa in September sparked weeks of protests across Mexico against corruption and violence.
Amnesty has described torture as being "out of control" in the Latin American nation amid a six-fold increase in the number of reported cases in the past decade.
The upsurge in cases of torture has coincided with the country's nine-year-long "war on drugs", which has also seen over 100,000 killings and over 22,000 disappearances, including a case of the 43 students, the human rights group said
Yo Soy 132 Londres' protests will be staged under the banner EPNnotWelcome and in their letter to the Queen, they demanded an independent investigation into the students' case and urgent action on the scale of other disappearances.
"We have to ask: Why, Madam? Why is the head of a government that has so little respect for the values that this nation upholds is invited to stay at Buckingham Palace?" the letter said.
Tom Davies, Amnesty International UK's Stop Torture campaign manager, said: "While President Pena Nieto is getting the red-carpet treatment, his police and security services have serious blood on their hands.
"With public officials complicit in drug cartel crimes as well as thousands of torture cases of its own, President Pena Nieto needs to radically overhaul his country's woeful response to this crisis."
He called on Mr Cameron and Deputy Prime Minister Nick Clegg to ensure that the issue of torture in Mexico was not passed over in silence.
Campaigners from Amnesty will today break open a traditional brightly-coloured papier-mache Mexican pinata filled with their petition outside the Mexican embassy in central London.
The incoming state visit takes place just four months after the Prince of Wales and the Duchess of Cornwall carried out an official tour to Mexico.
The country has the second-largest economy in Latin America and is a major oil producer and exporter. The president will travel to Aberdeen on Thursday to attend a reception with representatives from the oil and gas sector.
Mr Pena Nieto and his wife recently became embroiled it what became known as the "White House" scandal.
The First Lady - a glamorous ex-telenovela star - was in the process of paying for a lavish £4.4 million property when it was revealed that it was owned by a company which has benefited substantially from her husband's presidency.
She posted a video statement on her website to explain the situation, outlining her personal wealth from her acting career and announcing her decision to sell her stake in the mansion to defend "my integrity, that of my children, and my husband".
Ms Rivera starred in the hit soap opera Distilling Love and is known as the La Gaviota - the seagull - the name of the girl she played in the show.
Mr Pena Nieto, 48, came to power in 2012, representing the Institutional Revolutionary Party (PRI) which ruled Mexico for 71 years until 2000.
He vowed to focus on curbing violence and insisted he would not make pacts with organised crime.
Some refer to him by the nickname Bombon - sweetie - the name female admirers would scream at him during election rallies. A former governor of the state of Mexico, he was frequently dubbed Mexico's most handsome politician.
On Friday, Mexican police captured the country's most wanted drug lord, Servando "La Tuta" Gomez - a major coup for Mr Pena Nieto's administration.