Percy Shelley's best known response to Peterloo is his Masque of Anarchy, which he wrote very quickly after receiving news of the events, but which was not published until much later.  The sources on which he drew has recently been discussed by Phil Connell in 'A Voice from over the Sea: Shelley's Masque of Anarchy, Peterloo, and the English Radiical Press' (Review of English Studies 2019, vol. 70, No. 296, 716-731), showing an awareness of some of the more radical sectors of the English press than it has been customary to attribute to Shelley.  It also shows that Shelley himself remained more allied to the constitutional movement for Parliamentary reform than to any emerging insurrectionary wing. 

There is, however, a much shorter poem, probably written between mid October and mid December 1819, in which a rather different, more radical image is conjured in the last few lines.  There is no similar optimism about an insurrectionary spark in Shelley's correspondence from this period, although there may have been an element of prudence involved in that.  But it is plausible to think that it points to the knife edge on which people's sense of the situation balanced: that the government had far overstepped the bounds in its suppression of liberty, and that while few had the appetite for the measures, many increasingly felt that they would be justified.

When Mary Shelley edited Shelley's papers she gave the poem the title:

‘England in 1819’ 

An old, mad, blind, despised, and dying King;

Princes, the dregs of their dull race, who flow

Through public scorn,—mud from a muddy spring;

Rulers who neither see nor feel nor know,

But leechlike to their fainting Country cling

Till they drop, blind in blood, without a blow.

A people starved and stabbed on th’untilled field;

An army whom liberticide and prey

Makes as a two-edged sword to all who wield;

Golden and sanguine laws which tempt and slay;

Religion Christless, Godless, a book sealed;

A senate, Time's worst statute, unrepealed—

Are graves, from which a glorious Phantom may

Burst, to illumine our tempestuous day.

In Shelley's A Philosophical Review of Reform  also written in 1820, although not published until many years later, his commitment to gradual reform, rooted in a genuine enlightenment of the people is clear:

'A Republic, however just in its principle and glorious in its object, would through the violence and sudden change which must attend it, would incur a great risk of being as rapid in its decline as in its growth. It is better that they should be instructed in thewhole truth, that they should see the clear grounds of their rights, the objects to which they ought to tend; and be impressed with the just persuasion that patience and reason and endirance are the means of a calm yet irresistible progress.' (p. 68)

And 'Any sudden attempt at Universal Suffrage would produce an immature attempt at a Republic. It is better that an object so inexpressible great and sacred should never have been attampted than that it should be attempted and fail' (72)

A transciption of A Philosophical Review of Refor is available at:

http://oll-resources.s3.amazonaws.com/titles/2510/Shelley_PhilosophicalReform1623_Bk.pdf