The new force is made up of staff from existing agencies
The UK's new border agency has been officially launched - but ministers say further legislation is needed in the autumn before it is fully operational.
The UK Border Agency (UKBA) will eventually merge the work of 25,000 immigration, revenue and customs and visa staff in 135 countries.
Initially about 1,000 frontline staff in England and Wales will be given enhanced "police-like" powers.
The Tories and Lib Dems dismiss the change as a re-branding exercise.
The Home Office says the UKBA will be the "eyes and ears" of the police at the border, with a "wide range" of powers.
Its officers, who will be drawn from the Border and Immigration Agency, HM Revenue and Customs and UK visas, will work to combat smuggling, immigration crime and border tax fraud.
Over the next four months, 1,000 frontline staff in England and Wales will be conferred with both immigration and customs powers, enabling them to work more closely with the police by detaining suspects.
They will also have the right to board and search vehicles, aircraft and trains, and to enter premises, seize goods.
The government has promised legislation in the autumn for a full merger of customs and immigration officials' work, paving the way for 9,000 warranted officers operating in local communities, at the UK's borders and in 135 countries around the world.
But UKBA officers will have less wide-ranging powers in Scotland, where they will not be allowed to detain suspects.
Shadow immigration minister Damian Green, for the Conservatives, accused the government of "hyping up" the changes.
He said: "There is a lot less to this than the government is making out.
"It is basically the same people doing the same jobs as they were yesterday and the government by making these changes is admitting our border system clearly isn't good enough. It needs a much more radical change, actually to achieve anything very effective."
He said the new border force would do nothing to track down illegal migrants already in the UK and the firms who employ them.
"What we want is a much more effective border force but also a force that has the powers to chase people traffickers and employers of illegal labour. That's why we want a much more radical change."
He said the Conservatives would set up a dedicated border police force with the same powers to stop and search and detain suspects as the police.
Liberal Democrat home affairs spokesman Chris Huhne called the change a "headline-grabbing rebadging of existing functions".
"The staff of this Border Force-Lite will have "police-like" powers but will be restricted to immigration and customs offences only.
"If the Government was serious about border security they would provide 24-hour cover at all ports of entry to the UK and immediately reintroduce exit checks."
Home Secretary Jacqui Smith, who launched the agency on a visit to Gatwick airport, insisted the new force would work with immigration officials tracking down illegal immigrants already in the UK - and that it had sufficient powers to strengthen border controls.
"It will have tough customs and immigration and police-like powers and I think that will mean it is better-equipped than ever to guard our ports and airports, protect the country from illegal immigration, organised crime and terrorism," she told BBC Radio 4's Today programme.
The Conservatives have also criticised the government for failing to include police in the new organisation.
Ms Smith said the new agency would work "extremely closely" with the 3,000 police officers stationed at the UK's borders and would have a senior police officer on its board.
The government has reached an agreement with police in England, Wales and Northern Ireland to share intelligence and work more closely together.
But it has yet to reach a similar deal with the Association of Chief Police Officers in Scotland, where policing is a devolved matter.