London First said the strike is "totally unjustified" and punishes commuters.
David Leam, infrastructure director at business group London First, said: "The RMT's strike is totally unjustified. It punishes the many commuters unable to work from home as well as the self-employed who could be seriously out of pocket at the end of this week.
"It also undermines London's reputation as a great city to get around, visit and do business in. Union leaders should get back to the negotiating table urgently instead of showing such contempt for commuters."
Meanwhile, London Chamber of Commerce and Industry policy manager Siwan Puw said: “There is no doubt that passengers are becoming increasingly frustrated by the ongoing situation. It not only has a damaging economic impact, but continued public transport strikes do considerable damage to London’s international reputation.
“Given that we are facing the longest period of strike action since 1968 it’s now more important than ever that the department for transport and TfL work together to explore opportunities to drastically improve services from the south-east into London.”
Conservative MP for Lewes Maria Caulfield tweeted that the tide appears to have turned against the union in the dispute.
Judging by my inbox this morning the tide has turned against the unions in this industrial dispute. #southernstrike— Maria Caulfield MP (@mariacaulfield) August 8, 2016
A spokesperson for the mayor of London said: “Thousands of Londoners and longer-distance commuters will face further disruption as a result of this strike action. The government should accept Sadiq's offer of putting a senior team from Transport for London in charge of the GTR Southern franchise until we get a permanent resolution.”
Today Charles Horton, chief executive of Southern, said he is "deeply sorry" for any inconvenience caused.
"Many people's lives are being turned upside down by this strike action by the RMT trade union, which is unacceptable, it is unnecessary and it is unjustified," he told BBC radio 5.
The dispute is predominantly due to GTR's plan for Southern drivers, rather than guards, to open and close carriage doors.
The strike comes after the company was forced to bring in an emergency timetable, cutting 341 services a day, or 15 per cent of its trains.
RMT said this morning that just 393 members were balloted, with 321 voting, and 206 voting to strike.