(as translated by Shakir)
[72:18] And that the mosques are Allah's, therefore call not upon any one with Allah:
[72:19] And that when the servant of Allah stood up calling upon Him, they wellnigh crowded him (to death).
[72:20] Say: I only call upon my Lord, and I do not associate any one with Him.
[72:21] Say: I do not control for you evil or good.
[72:22] Say: Surely no one can protect me against Allah, nor can I find besides Him any place of refuge:
[72:23] (It is) only a delivering (of communications) from Allah and His messages; and whoever disobeys Allah and His Messenger surely he shall have the fire of hell to abide therein for a long time.
In Surat al-Fatihah, we as Muslims regularly recite
iyyaka nasta'een, translated as "You [i.e. God] alone we ask for help..." In addition, all of the verses in the Qura'n where du'a is asked is addressed to Allah alone (e.g.
rabbi zid ni 'ilma in Surat Ta-Ha).
As such, it seems rather clear to me that invocation is to be for God alone, and that calling upon others is a form of shirk.
I find that amongst modern Shi'a (I can't comment on historical Shi'ism) and Barelvi Sufis there are many who seem to go against this idea. Using the Shi'a as an example, many of them directly invoke Ali for help (e.g. "Ya Ali madad"), which to me is no different than a Christian saying "Jesus is our Savior".
How can such behaviour be justified Islamically?
Please do not confuse the focus of this question with that of wasilah, as the issue of wasilah IS DIFFERENT than direct invocation to other then ALLAH swt. The Qur'an does command to seek a LEGAL wasilah (i.e through good deeds and following Sunnah and naufil of the prophet pbuh)