A test case is due to take place at the High Court
Retired Gurkhas have been protesting outside Parliament against the government's refusal to allow them the right to live in the UK.
The rally is a forerunner to a legal challenge to their immigration status at the High Court in London on Tuesday.
Gurkhas who retired from the British Army after the regiment moved its main base from Hong Kong to the UK in 1997, have automatic permission to remain.
But those who left earlier must apply and can be refused and deported.
About 30 Gurkhas have gathered to protest outside the Houses of Parliament, where they will hand over a petition to Downing Street before walking to the nearby Gurkha Memorial.
Campaigner Peter Carroll told the BBC it was important to "remind the government that thousands of brave and loyal Gurkhas who have fought for Britain are forced to go to court to ask for something the vast majority of British people think they should have... the right to live in the UK".
Tuesday's test case is being brought by five veteran Gurkhas and the widow of a veteran.
The outcome of the case could affect more than 2,000 retired Gurkhas who have been denied the right to settle in the UK.
The Gurkhas' legal team argue that foreign soldiers in the British army would normally be eligible to settle in the UK after completing four years' service anywhere in the world.
But Gurkhas who retired before 1997 "continue to be denied the opportunity to obtain settlement on the same basis as foreign soldiers discharged in the same period".
Their lawyers say this amounts to "unlawful discrimination".
They added some veterans have died waiting for immigration appeals while others are seriously ill.
The Home Office said the change in rules was introduced because the soldiers were only likely to have developed strong ties to Britain once the Gurkhas became a UK-based force. Some protesters are also calling for pension rules to be reformed in a similar way.
Actress Joanna Lumley's father served with the Gurkhas
Soldiers who retire after July 1997 - when Hong Kong, the former base of the Gurkhas, was handed over to China - receive a pension on the same terms as the rest of the British Army.
But those who retired before that date collect one-sixth of the amount.
Actress Joanna Lumley, whose father served with the Gurkhas, will be among the protesters expected to gather outside the High Court on Tuesday.
She said: "Like so many people in Britain I am ashamed at how successive governments have failed these magnificent and loyal soldiers."
"The overwhelming wish of the British is to allow them to live here with us if they so choose."