Blake's poem "London" (1794) is perhaps his strongest condemnation of British urban society at the end of the 18thC.
Because Blake was born and lived in London his entire life, he saw firsthand the difference between the London of the mid-18thC. and the city as it was at the beginning of the Industrial Revolution--and he didn't like what London represented.
In the first stanza, for example, Blake compares London's streets to the Thames River:
I wander thro' each charter'd street,/Near where the charter'd Thames does flow. . . .
He is comparing the river, which is navigated with charts, to London's streets, which now are regulated so that there is no longer any freedom left. The image created by "charter'd" is meant to create a sense of constriction and impaired freedom. This lack of freedom (or, another way, too much restriction) has created a population whose faces show only "marks of weakness, marks of woe."
The lack of freedom, according to Blake, is not imposed upon the citizens by an outside force. Rather, the restrictions that everyone feels are the result not of an outside force but "the mind-forg'd manacles" of each individual. In other words, the city's woes have been created by the inhabitants themselves, from the highest to the lowest. Implicitly, if one has the ability to create manacles in the mind, one has the capability of losing those manacles by an act of self will.
The third stanza constitutes an indictment of both the church and king:
How the Chimney-sweeper's cry/Every black'ning Church appalls;/And the hapless Soldiers sigh/Runs in blood down Palace walls.
Blake condemns the church for not providing comfort to the poorest of the society--represented by the chimney sweeps--and points out the the government (the "Palace") doesn't really care about the fate of the soldiers that protect its interests. This is a protrait of a society in which institutions that should care for the people have completely abandoned them. When Blake uses "every black'ning Church," it's quite likely that he means two things: the churches in London are black with soot from coal fires, and, more important, the church has blackened the souls of London's citizens by ignoring their plight.
Leaving his most bitter criticism of society for the fourth stanza, Blake argues here that the London's most serious problem is the influx of prostitutes, women who have been displaced by the Industrial Revolution, whose activities blast "the new born Infant's tear,/And blights with plagues the Marriage hearse." Here, Blake refers to the transmission of sexually-transmitted diseases from the men who patronize prostitutes to their wives, who then transmit the diseases to their newborns.
Ultimately, the poem's subject is the destruction of a society brought about, in part, by each citizen's complete inability to correct any of the society's problems and by the ruling class, represented by the church and the government.
William Blake critiques several aspects of his society. Perhaps the best known of his poems concerning child labour is "The Chimney Sweeper", which evokes the cruelty of young children being forced to sweep chimneys:
When my mother died I was very young,
And my father sold me while yet my tongue,
Could scarcely cry weep weep weep weep,
So your chimneys I sweep & in soot I sleep.
In "London", Blake observe multiple social ills and blames them on the prevalence of commerce. The human misery he sees, he identifies with "charter'd" (commercial) streets. He references in the poems the suffering of young chimney sweeps, soldiers, infants, and prostitutes. He sees the commercialization of Britain and its "dark Satanic mills" as corrupting interpersonal relationships and religion. He is particularly concerned with preserving the innocence of childhood.