Up to 150 disability rights campaigners have been protesting outside IT firm Atos, which carries out "fit for work" assessments for the government.
Campaigners say its tests for people on disability allowances are "damaging and distressing" and have led to suicides.
Atos, a sponsor of the Paralympic Games, said it ensures its service is "professional" and "compassionate".
Protesters also targeted the Department for Work and Pensions in Westminster, where one person was arrested.
The small break-away group, including two wheelchair users, blocked the doors of the office in a "surprise secret action", UK Uncut said.
Protesters who got inside the DWP building have since been removed. The Metropolitan Police said one person was arrested for breach of the peace.
Demonstrations have been held throughout the week, culminating in an attempt to bring the French firm's headquarters in Triton Square, Marylebone, to a close.
Earlier Paralympics chief Lord Coe defended Atos and said the company had helped with the London 2012 accreditation process, recruitment of volunteers, and results delivery service.
"They're partners and they help us to deliver this... Their work goes way beyond any of that visibility."
The government - which makes decisions based on Atos Work Capability Assessments - has said it is trying to control the cost of disability allowances.
It says more than £600m a year is being spent on overpayments to people who no longer qualify for the level of benefits they are receiving.
It has said checks are important to make sure the benefit system supports people and does not trap them.
Claire Glasman, Winvisible campaigner, has cerebral palsy and is on incapacity benefit.
She said her group has campaigned for a mother with a spinal injury who was injured at work and found unfit to work but scored zero on her assessment meaning she must apply for Job Seeker's Allowance, find waged work or rely on relatives.
Ms Glasman said: "We don't know how people are managing.
"Women are particularly affected because we are the ones most likely to be doing unwaged caring work as well as coping with severe health problems."
John Williams who has Crohn's Disease, epilepsy and a number of other conditions joined about 200 campaigners.
He said he is having to live on benefits of £500 a month and rely on liquid food drinks.
He said: "I want a fair system. It's absolutely appalling. It feels like Atos is working with the government to get the numbers down."
Roger Lewis from Disabled People Against Cuts told BBC Five Live assessments were causing "huge damage and distress to disabled people".
He said: "We now have a situation where we know that people have gone through the Atos assessments who have unfortunately died as a result. Some have committed suicide. Some have had heart attacks."
He said a parliamentary group of MPs has been looking at coroners' reports where Work Capability Assessments have been cited as a contributing factor.
The DWP said it did not have research data on the impact of assessments on people's mental health and that no links could accurately be made to suicides.
A spokesman said 15% of "fit for work" decisions were overturned on appeal and the system was subject to an annual review.
He said: "Since 2010 we have considerably improved the Work Capability Assessment process.
"As a result we are seeing an increase in the number of severely disabled people being given long-term unconditional support."
A spokeswoman for the French multinational Atos said: "We fully respect people's right to peaceful protest and we understand this is a highly emotive issue.
"We do not make decisions on people's benefit entitlement or on welfare policy but we will continue to make sure the service that we provide is as highly professional and compassionate as it can be.
"We do this through a constant programme of training and education for our staff, a rigorous recruitment process for healthcare professionals and those going through the process on the ground."