What does it mean?
1. What does 'spy another blow' here mean?

The Prevention of Terror Act gives a government panel the right to imprison terror suspects for two years, with multiple extensions, or restrict their movements for five years. Critics spy another blow to civil liberties, which were already under siege.

2. What is 'sweeping power' ?

The government has produced no evidence that such sweeping powers are warranted, the International Commission of Jurists, a human-rights group, claims. It laments that only one member of the government’s detention panel need have legal experience.

3. What does 'tacking right only to head off leadership challenges from even less palatable parts' mean?

Supporters plead that the prime minister is tacking right only to head off leadership challenges from even less palatable parts of his party (on April 2nd Mahathir Mohamad, an influential former prime minister, renewed his call for Mr Najib to step down). But that is not much comfort to anyone.http://www.economist.com/news/asia/21648027-anti-terror-law-curtails-liberties-lurch-illiberalism

Question

What does it mean?
1. What does 'spy another blow' here mean?

The Prevention of Terror Act gives a government panel the right to imprison terror suspects for two years, with multiple extensions, or restrict their movements for five years. Critics spy another blow to civil liberties, which were already under siege.

2. What is 'sweeping power' ?

The government has produced no evidence that such sweeping powers are warranted, the International Commission of Jurists, a human-rights group, claims. It laments that only one member of the government’s detention panel need have legal experience.

3. What does 'tacking right only to head off leadership challenges from even less palatable parts' mean?

Supporters plead that the prime minister is tacking right only to head off leadership challenges from even less palatable parts of his party (on April 2nd Mahathir Mohamad, an influential former prime minister, renewed his call for Mr Najib to step down). But that is not much comfort to anyone.http://www.economist.com/news/asia/21648027-anti-terror-law-curtails-liberties-lurch-illiberalism

Answer

1) "To spy" means "to see". "A blow" is a heavy hit or strike.

2) The keyword is "sweeping" - it means "covering everything".

3) I don't know how the politics work here, ie. whether "right" means "right-wing". Anyway, "to tack" means to steer a boat in a certain direction. "To head off X" means to move quickly to stop X". The "less palatable parts of his party" are the ministers who would do a worse job.