The jail population in the United States is expected to reach 2 million on Tuesday, prompting a wave of protests across America.

An explosion in inmate numbers in recent years means that although the US makes up 5% of the total global population, it now accounts for 25% of the world's prisoners.

Civil rights activists and penal reform campaigners are staging vigils in more than 30 cities to protest at the reaching of the unwelcome milestone.

They are demanding changes in sentencing policies to reduce the numbers of non-violent offenders, particularly drug users, who are incarcerated.

About 1.3 million of the current jail population have been imprisoned for non-violent crimes, usually drug offences.

The Justice Policy Institute estimates that the total prisons bill will increase to $41bn this year, a rise of almost �2bn on 1999.

Much of this is due to the scores of extra prisons that have had to be built to cope with the rise of "three strikes and you are out" sentencing policies which have introduced long fixed jail terms for repeat offenders.

Racist justice claims

Campaigners are particularly concerned at the high proportion of black people behind bars, which they say is often due to racist attitudes within the police and the judicial system as well as social deprivation.

Studies have shown that in cities such as Baltimore and Washington at least half of all young black men are either in prison, on parole or on probation.

This has also led to an alarming growth in the number of black people who have lost the right to vote because of their criminal convictions.

Reformers warn that in the states with the most restrictive voting laws, 40% of African American men could soon be permanently disenfranchised in this way.

Call for reform

Jenny Gainsborough, of the research organisation, Sentencing Project, said she hoped Tuesday's protests would highlight the need for reform to stop the upward spiral in the prison population.

"We just want people to be aware of the huge numbers of people who are being locked up and the contrast between the US and other western industrialised democracies.

"Beyond that, we want people to start calling for reform and to talk to their representatives, and to ask for changes, partly in the drug laws but in other sentencing too, and to provide alternative sanctions for non-violent criminals who don't need to be locked up in prison for long periods of time."

However, the reformers fear they may struggle to be heard in an election year when none of the presidential hopefuls will want to leave themselves open to claims that they are soft on crime.