They accused BNP of seeking to create a "messy monster of a bank" which would set the French industry back six years.
The managers, headed by SG's global head of equities Yves Tuloup, said they had given up the traditional Bastille day holiday to bring their message to the City because, with 48 per cent of the bank's shares in non-French hands "that is where the shareholders are".
They said they would be meeting shareholders over the next few days to convince them BNP's rival plans for a three-way merger would end in "disaster".
Mr Tuloup, who was at pains to distance himself from the bank's board, which has campaigned aggressively against BNP's bid, insisted the campaign group he had founded, called Action Against the BNP Raid, was "not about job protection".
"We are shareholders too," he said, pointing out that 31,000 SG employees have an average of two years' salary tied up in the bank's shares, while many senior managers have considerably more. "We are not against retail consolidation - we just want to be able to choose our partner."
He said the group had decided to bring its message to the City because, with 48 per cent of the bank's shares in non-French hands, "that is where the shareholders are".
He laid into BNP and its chairman Michel Pebereau claiming that the bank lagged SG both in terms of product innovation and efficiency.
SG staff contributed 40 per cent more in profit per head than their corresponding BNP employees, he said.
On the basis of a customer survey carried out by supporters within the bank, he claimed that 20 per cent of its customers would move if the BNP takeover went ahead. "The time has come to say that this is a project made of illusions that will lead to disaster," he said.
"If BNP were successful it would create a messy Franco-French monster of a bank, so messy, so Franco-French it would be unable to seize the opportunities in Europe we have created for Societe Generale over the past six years."
M Tuloup was joined by Patrick Pagni, the former head of SG's UK brokerage Strauss Turnball, who is now head of strategy for the investment bank, and Jean-Marie Avinaud, head of strategic research for the SG retail bank.