Police intel suggested Romanian adults had been smuggled to the Kingsbury area to work on building sites as unskilled labourers.
Met officers and the Romania Police executed a number of warrants in early morning raids on Tuesday targeting the criminal modern slavery operation.
Four warrants were executed across Harrow and Brent with officers also finding large quantities of cash and a firearm.
Eight men have been arrested in London for human trafficking. They have been taken to a north London police station.
A further five men were arrested in Romania where gold and around €250,000 was seized.
Those arrested are aged between 22 and 49-years-old. They have been arrested for a variety of offences, including causing another to complete forced or compulsory labour and human trafficking contrary to the modern slavery act 2015.
Detective Constable Dec Wilson from Central Specialist Crime, said: “These arrests in the UK and abroad are the result of working closely with our partners in Romania for the past 18 months, to identify those involved in the human trafficking and labour exploitation.
“We need the public’s help as they have an important role to play in recognising and reporting modern slavery. If you suspect someone may be a victim of modern slavery, report it. You will always be taken seriously, and protection and support is available.
“Often those affected do not see themselves as potential victims of labour exploitation and many will have been coerced into this life to make money for an organised crime network.
“We believe there are victims of modern slavery in every borough across London and the public may encounter them every day, possibly without realising. As well as being exploited for labour, victims have been found working in construction, domestic servitude, agriculture, cannabis factories and in places you use yourself, such as car washes, barbers and nail bars.
“Victims are often told the police and authorities in the UK are not to be trusted and with limited English the victims are unable to seek help, even if they want to.”
If you suspect that you, or someone you have come into contact with, may be a victim of modern slavery or trafficking and require support, please call The Salvation Army’s 24 hour confidential referral helpline on 0800 808 3733. This is the best way to get support to anyone you suspect might be a victim.
You can also report a suspicion or seek advice through the Modern Slavery Helpline confidentially on 08000 121 700. This is open 24 hours a day, 365 days a year. You can also report to the police online at www.met.police.uk or by calling 101, in case of an emergency dial 999. Alternatively, you can contact the independent charity Crimestoppers anonymously on 0800 555 111 or online at crimestoppers-uk.org.