What are the strategies used by the poet in the poem "London"?

I wander thro' each charter'd street,

Near where the charter'd Thames does flow. 

And mark in every face I meet

Marks of weakness, marks of woe.

In every cry of every Man,

In every Infants cry of fear,

In every voice: in every ban,

The mind-forg'd manacles I hear 

How the Chimney-sweepers cry

Every blackning Church appalls, 

And the hapless Soldiers sigh

Runs in blood down Palace walls 

But most thro' midnight streets I hear

How the youthful Harlots curse

Blasts the new-born Infants tear 

And blights with plagues the Marriage hearse 

The poem is written in such a way that the emphasis on certain words or syllables in each line seems to represent a man walking--that is, the speaker himself. There are four stressed word or syllables in each line. For example: 

HOW the CHIMney-SWEEPers CRY
EVery BLACKning CHURCH apPALLS,

AND the HAPless SOLDiers SIGH
RUNS in BLOOD down PALace WALLS

This walking effect can be seen in every line. The walk is leading up to the most terrible image of all, which Blake introduces by saying, "But most..."

But MOST through MIDnight STREETS i HEAR
HOW the YOUTHful HARlots CURSE

There is a heavy emphasis in each of the three words, "youthful harlots curse," because Blake thinks this is the worst spectacle to be seen and heard in London. The harlots could be very youthful indeed. Many might still be children. It is because so many of them are so young that the image is so terrible. The entire poem seems to be marching towards that image in the last stanza. The youthful harlots are destined to get syphilis and other sexually transmitted diseases, and these can be passed on to many men, who can pass them on to their innocent wives when they get married, and the women's newborn infants can be born with a disease that will grow with them as they mature, since those diseases were incurable. That is why he says that the youthful harlots curse

Blasts the new-born Infants tear
And blights with plagues the Marriage hearse

There are disease germs even in the tears shed by the infant the moment it is born. The word "hearse" does not mean a vehicle for transporting corpses but a carriage taking the bride and groom away from the church and off on their honeymoon. The groom can be infected with syphilis from consorting with the youthful prostitutes, and he will soon be passing it on to his bride, who will then be likely to give birth to an infected baby.

Note that the same walking effect, with four words or syllables emphasized in each line, is continued to the very end:

BLASTS the NEW-born INfants TEAR
And BLIGHTS with PLAGUES the MARRiage HEARSE

It is of incidental interest that Blake's poem "London" resembles the opening lines of T. S. Eliot's "The Love Song of J. Alfred Prufrock" in mood if not in meter.

Let us go then, you and I,
When the evening is spread out against the sky
Like a patient etherized upon a table;
Let us go through certain half-deserted streets,
The muttering retreats
Of restless nights in one-night cheap hotels
And sawdust restaurants with oyster shells:
Streets that follow like a tedious argument
Of insidious intent