PSL/ANSWER in the US and SWP/Stop The War Coalition in the UK are always very quick off the ground to out up Facebook events for city centre anti-war protests. They then flood them with branded placards and hoover up contact details via 'petitions'.
In the immediate aftermath of the Grenfell Tower fire, the SWP called a central London demo, competing with the local demo called by people actually affected by the fire. Later, local residents tore the SWP logos off the mass produced placards.
By mass printing placards and leaving them around for people to pick up, they make their orgs look ten times the size. 
The goal of the march for these orgs is to corral people to a static stage with a PA system to listen to their chosen list of speakers. This sometimes doubles up as one side of a police kettle so they really get a captive audience.
They often get speakers from outside the organisation, Stop the War and ANSWER are front groups specifically to be able to attract a broader group of people than Trotskyist/Marcyite sects can. If you see a speaker list, try contacting people on it, they might not know.
If you go to marches, take your own placard, or something you can stick onto an SWP/ANSWER one to 'customize' it. Can also encourage people to rip off or cover the branding.
On petitions, instead of your name, consider putting 'Google comrade delta' so that people who might give these orgs their personal data know what they're getting into (TW rape)
socialistunity.com/swp-party-memb…
socialistunity.com/swp-party-memb…
Background here on the ANSWER coalition, written 2005 for the magazine of the War Resisters League.
libcom.org/library/politi…
libcom.org/library/politi…
Lots of background on the SWP here libcom.org/tags/swp
Example.
This account of PSL and similar orgs was written by ex-members. They will recruit young people who want to take action, but most of those people leave disillusioned and burnt out.
libcom.org/library/where-…
libcom.org/library/where-…
The challenge here is not to organise a 'better' version of these rallies that can recruit people to 'better' sects, but to make the connection from the war mobilisation to people's every day conditions of life, and to fight against those uncompromisingly
And also, if you are able to get a large number of people together specifically to oppose the war, why not do it somewhere it'll have a direct impact?
Examples of SWP communications with the police prior to an anti-fascist demonstrationin East London.
