At Southwark Cathedral, a service of commemoration will be held to remember the victims of the terrorist attack.
The Home Office warned that Britain faces a severe threat from Islamist terrorism for at least another two years.
MI5 and counter-terrorism police are currently running more than 500 live operations involving roughly 3000 "subjects of interest" at any one time.
However, more than 20,000 people who have previously been investigated and categorised as a "closed subject of interest" could still pose a threat.
Salman Abedi was categorised as one such individual at the time of his attack at the Manchester Arena last year, which resulted in 22 people being killed and hundreds injured.
A report by David Anderson QC into four of the five attacks of 2017 concluded that the Manchester Arena attack "might have been averted" if two pieces of intelligence about Abedi had been interpreted differently by MI5.
The new security measures to be unveiled are expected to focus on the importance of trying to spot those like Abedi who may have become radicalised again.
An extra 1000 security services staff will also be recruited to collect and analyse data as well as keep suspects under better surveillance.
MI5 will also be expected to share information about suspected extremists more widely with other organisations, including local councils and neighbourhood policing units.
Extra resources will be provided for areas such as Birmingham, Bradford and Manchester to help monitor Muslim extremists.
Ahead of the commemorative events, which will also include a minute's silence to remember the eight people murdered, Prime Minister Theresa May said that the capital's resolve against terrorism had never been stronger.
"My message to those who seek to target our way of life or try to divide us is clear - our resolve to stand firm and overcome this threat together has never been stronger," the Prime Minister said.
Security agencies and police have foiled 12 Islamist and four extreme right-wing plots since March last year.
