In its final declaration at the close of the congress on Monday, the group called on Syrians to "break down the barriers of fear" and overturn the government of Bashar al-Assad, the Syrian president, but not through violent means.
The front, created just over two months ago by around 50 opponents including Abdel Halim Khaddam, the outspoken former vice-president, and Ali Sadreddin al-Bayanouni, Muslim Brotherhood leader, said the Syrian government had "lost all the reasons for continued survival".
It urged Syrian citizens to remember the courage of their ancestors who won independence for the country in 1946.
Denouncing the government as "unlawful" and based on coercion, the front said it aimed to create a "modern, democratic and civil state in Syria, based on plurality (and) peaceful transfer of power ... where the free and fair ballot box is the expression of the general collective will.
"The National Salvation Front also invites the armed forces to shoulder national and historic responsibility, to be the army of the people," the statement added.
It urged the security forces, especially "officers used by the regime to inflict injustice on the citizens ... to live up to their moral and national responsibilities and refuse to obey the orders of the corrupt group that all of you know is corrupt and oppressive."
Alongside Khaddam and the Muslim brotherhood, Kurdish parties, independents and communists took part in the London meeting.