Staff are being advised to stay in bed or work from home on 1 May when
anti-capitalist protesters target the capital.
A spokesman for BT said, "We are giving common sense advice to staff. If you do have facilities to work from home, it is a day to do so."
BT staff who intend to come into the city are being told not to carry their laptops or speak loudly on their mobile phones.
"Shouting 'Buy! Buy! Buy!' into your mobile is not a good idea," said the spokesman.
Dixons refused to comment on stay-at-home tactics. A spokeswoman said, "We are not giving any information out. We found from past experience that the more information given out the more likely you are to be a target. We are closing the doors on this one."
A spokeswoman for the Home Office said that telling employees to remain at home is "essentially a matter for individual businesses concerning what procedures or precautions they put in place."